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A media type (also known as a Multipurpose Internet Mail Extensions or MIME type) is a standard that indicates the nature and format of a document, file, or assortment of bytes. It is defined and standardized in IETF's RFC 6838.

The Internet Assigned Numbers Authority (IANA) is responsible for all official MIME types, and you can find the most up-to-date and complete list at their Media Types page.

Important: Browsers use the MIME type, not the file extension, to determine how to process a URL, so it's important that web servers send the correct MIME type in the response's Content-Type header. If this is not correctly configured, browsers are likely to misinterpret the contents of files and sites will not work correctly, and downloaded files may be mishandled.

Structure of a MIME type

The simplest MIME type consists of a type and a subtype; these are each strings which, when concatenated with a slash (/) between them, comprise a MIME type. No whitespace is allowed in a MIME type:

type/subtype

The type represents the general category into which the data type falls, such as video or text. The subtype identifies the exact kind of data of the specified type the MIME type represents. For example, for the MIME type text, the subtype might be plain (plain text), html (HTML source code), or calendar (for iCalendar/.ics) files.

Each type has its own set of possible subtypes, and a MIME type always has both a type and a subtype, never just one or the other.

An optional parameter can be added to provide additional details:

type/subtype;parameter=value

For example, for any MIME type whose main type is text, the optional charset parameter can be used to specify the character set used for the characters in the data. If no charset is specified, the default is ASCII (US-ASCII) unless overridden by the user agent's settings. To specify a UTF-8 text file, the MIME type text/plain;charset=UTF-8 is used.

MIME types are case-insensitive but are traditionally written in lowercase, with the exception of parameter values, whose case may or may not have specific meaning.

Types

There are two classes of type: discrete and multipart. Discrete types are types which represent a single file or medium, such as a single text or music file, or a single video. A multipart type is one which represents a document that's comprised of multiple component parts, each of which may have its own individual MIME type; or, a multipart type may encapsulate multiple files being sent together in one transaction. For example, multipart MIME types are used when attaching multiple files to an email.

Discrete types

The discrete types currently registered with the IANA are:

applicationList at IANA
Any kind of binary data that doesn't fall explicitly into one of the other types; either data that will be executed or interpreted in some way or binary data that requires a specific application or category of application to use. Generic binary data (or binary data whose true type is unknown) is application/octet-stream. Other common examples include application/pdf, application/pkcs8, and application/zip.
audio List at IANA
Audio or music data. Examples include audio/mpeg, audio/vorbis.
example
Reserved for use as a placeholder in examples showing how to use MIME types. These should never be used outside of sample code listings and documentation. example can also be used as a subtype; for instance, in an example related to working with audio on the web, the MIME type audio/example can be used to indicate that the type is a placeholder and should be replaced with an appropriate one when using the code in the real world.
font List at IANA
Font/typeface data. Common examples include font/woff, font/ttf, and font/otf.
image List at IANA
Image or graphical data including both bitmap and vector still images as well as animated versions of still image formats such as animated GIF or APNG. Common examples are image/jpeg, image/png, and image/svg+xml.
model List at IANA
Model data for a 3D object or scene. Examples include model/3mf and model/vml.
text List at IANA
Text-only data including any human-readable content, source code, or textual data such as comma-separated value (CSV) formatted data. Examples include text/plain, text/csv, and text/html.
video List at IANA
Video data or files, such as MP4 movies (video/mp4).

For text documents without a specific subtype, text/plain should be used. Similarly, for binary documents without a specific or known subtype, application/octet-stream should be used.

Multipart types

Multipart types indicate a category of document broken into pieces, often with different MIME types; they can also be used — especially in email scenarios — to represent multiple, separate files which are all part of the same transaction. They represent a composite document.

With the exception of multipart/form-data, used in the POST method of HTML Forms, and multipart/byteranges, used with 206 Partial Content to send part of a document, HTTP doesn't handle multipart documents in a special way: the message is transmitted to the browser (which will likely show a "Save As" window if it doesn't know how to display the document).

There are two multipart types:

message List at IANA
A message that encapsulates other messages. This can be used, for instance, to represent an email that includes a forwarded message as part of its data, or to allow sending very large messages in chunks as if it were multiple messages. Examples include message/rfc822 (for forwarded or replied-to message quoting) and message/partial to allow breaking a large message into smaller ones automatically to be reassembled by the recipient.
multipart List at IANA
Data that is comprised of multiple components which may individually have different MIME types. Examples include multipart/form-data (for data produced using the FormData API) and multipart/byteranges (defined in RFC 7233: 5.4.1 and used with HTTP's 206 "Partial Content" response returned when the fetched data is only part of the content, such as is delivered using the Range header).

Important MIME types for Web developers

application/octet-stream

This is the default for binary files. As it means unknown binary file, browsers usually don't execute it, or even ask if it should be executed. They treat it as if the Content-Disposition header was set to attachment, and propose a "Save As" dialog.

text/plain

This is the default for textual files. Even if it really means "unknown textual file," browsers assume they can display it.

Note that text/plain does not mean "any kind of textual data." If they expect a specific kind of textual data, they will likely not consider it a match. Specifically if they download a text/plain file from a <link> element declaring a CSS file, they will not recognize it as a valid CSS file if presented with text/plain. The CSS mime type text/css must be used.

text/css

CSS files used to style a Web page must be sent with text/css. If a server doesn't recognize the .css suffix for CSS files, it may send them with text/plain or application/octet-stream MIME types. If so, they won't be recognized as CSS by most browsers and will be ignored.

text/html

All HTML content should be served with this type. Alternative MIME types for XHTML (like application/xhtml+xml) are mostly useless nowadays.

Note: Use application/xml or application/xhtml+xml if you want XML’s strict parsing rules, <![CDATA[…]]> sections, or elements that aren't from HTML/SVG/MathML namespaces.

text/javascript

Per the HTML specification, JavaScript files should always be served using the MIME type text/javascript. No other values are considered valid, and using any of those may result in scripts that do not load or run.

For historical reasons, the MIME Sniffing Standard (the definition of how browsers should interpret media types and figure out what to do with content that doesn't have a valid one) allows JavaScript to be served using any MIME type that essentially matches any of the following:

  • application/javascript
  • application/ecmascript
  • application/x-ecmascript 
  • application/x-javascript 
  • text/javascript
  • text/ecmascript
  • text/javascript1.0 
  • text/javascript1.1 
  • text/javascript1.2 
  • text/javascript1.3 
  • text/javascript1.4 
  • text/javascript1.5 
  • text/jscript 
  • text/livescript 
  • text/x-ecmascript 
  • text/x-javascript 

Note: Even though any given user agent may support any or all of these, you should only use text/javascript. It's the only MIME type guaranteed to work now and into the future.

Some content you find may have a charset parameter at the end of the text/javascript media type, to specify the character set used to represent the code's content. This is not valid, and in most cases will result in a script not being loaded.

Image types

Files whose MIME type is image contain image data. The subtype specifies which specific image file format the data represents. Only a few image types are used commonly enough to be considered safe for use on web pages:

MIME type Image type
image/gif GIF images (lossless compression; superseded by PNG; basic animation support available)
image/jpeg JPEG images
image/png PNG images
image/webp WebP images
image/svg+xml SVG images (scalable vector images)
image/x-icon Windows icons

Audio and video types

Like images, HTML doesn't define supported types for the <audio> and <video> elements, so only some can be used on the Web. Media formats supported by the HTML audio and video elements explains both the codecs and container formats which can be used.

The MIME type of audiovisual files mostly indicate the container formats. The most common ones on the Web are:

MIME type Audio or video type
audio/wave
audio/wav
audio/x-wav
audio/x-pn-wav
An audio file in the WAVE container format. The PCM audio codec (WAVE codec "1") is often supported, but other codecs have limited support (if any).
audio/webm An audio file in the WebM container format. Vorbis and Opus are the most common audio codecs.
video/webm A video file, possibly with audio, in the WebM container format. VP8 and VP9 are the most common video codecs; Vorbis and Opus the most common audio codecs.
audio/ogg An audio file in the OGG container format. Vorbis is the most common audio codec used in such a container.
video/ogg A video file, possibly with audio, in the OGG container format. Theora is the usual video codec used within it; Vorbis is the usual audio codec.
application/ogg An audio or video file using the OGG container format. Theora is the usual video codec used within it; Vorbis is the usual audio codec.

multipart/form-data

The multipart/form-data type can be used when sending the values of a completed HTML Form from browser to server.

As a multipart document format, it consists of different parts, delimited by a boundary (a string starting with a double dash --). Each part is its own entity with its own HTTP headers, Content-Disposition, and Content-Type for file uploading fields.

Content-Type: multipart/form-data; boundary=aBoundaryString
(other headers associated with the multipart document as a whole)

--aBoundaryString
Content-Disposition: form-data; name="myFile"; filename="img.jpg"
Content-Type: image/jpeg

(data)
--aBoundaryString
Content-Disposition: form-data; name="myField"

(data)
--aBoundaryString
(more subparts)
--aBoundaryString--

The following <form>:

<form action="http://localhost:8000/" method="post" enctype="multipart/form-data">
  <label>Name: <input name="myTextField" value="Test"></label>
  <label><input type="checkbox" name="myCheckBox"> Check</label>
  <label>Upload file: <input type="file" name="myFile" value="test.txt"></label>
  <button>Send the file</button>
</form>

will send this message:

POST / HTTP/1.1
Host: localhost:8000
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:50.0) Gecko/20100101 Firefox/50.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Connection: keep-alive
Upgrade-Insecure-Requests: 1
Content-Type: multipart/form-data; boundary=---------------------------8721656041911415653955004498
Content-Length: 465

-----------------------------8721656041911415653955004498
Content-Disposition: form-data; name="myTextField"

Test
-----------------------------8721656041911415653955004498
Content-Disposition: form-data; name="myCheckBox"

on
-----------------------------8721656041911415653955004498
Content-Disposition: form-data; name="myFile"; filename="test.txt"
Content-Type: text/plain

Simple file.
-----------------------------8721656041911415653955004498--

multipart/byteranges

The multipart/byteranges MIME type is used to send partial responses to the browser.

When the 206 Partial Content status code is sent, this MIME type indicates that the document is composed of several parts, one for each of the requested ranges. Like other multipart types, the Content-Type uses a boundary to separate the pieces. Each piece has a Content-Type header with its actual type and a Content-Range of the range it represents.

HTTP/1.1 206 Partial Content
Accept-Ranges: bytes
Content-Type: multipart/byteranges; boundary=3d6b6a416f9b5
Content-Length: 385

--3d6b6a416f9b5
Content-Type: text/html
Content-Range: bytes 100-200/1270

eta http-equiv="Content-type" content="text/html; charset=utf-8" />
    <meta name="vieport" content
--3d6b6a416f9b5
Content-Type: text/html
Content-Range: bytes 300-400/1270

-color: #f0f0f2;
        margin: 0;
        padding: 0;
        font-family: "Open Sans", "Helvetica
--3d6b6a416f9b5--

Importance of setting the correct MIME type

Most web servers send unrecognized resources as the application/octet-stream MIME type. For security reasons, most browsers do not allow setting a custom default action for such resources, forcing the user to save it to disk to use it.

Some common incorrect server configurations:

  • RAR-compressed files. In this case, the ideal would be the true type of the original files; this is often impossible as .RAR files can hold several resources of different types. In this case, configure the server to send application/x-rar-compressed.

  • Audio and video. Only resources with the correct MIME Type will be played in <video> or <audio> elements. Be sure to use the correct type for audio and video.

  • Proprietary file types. Avoid using application/octet-stream as most browsers do not allow defining a default behavior (like "Open in Word") for this generic MIME type. A specific type like application/vnd.mspowerpoint lets users open such files automatically in the presentation software of their choice.

MIME sniffing

In the absence of a MIME type, or in certain cases where browsers believe they are incorrect, browsers may perform MIME sniffing — guessing the correct MIME type by looking at the bytes of the resource.

Each browser performs MIME sniffing differently and under different circumstances. (For example, Safari will look at the file extension in the URL if the sent MIME type is unsuitable.) There are security concerns as some MIME types represent executable content. Servers can prevent MIME sniffing by sending the X-Content-Type-Options header.

Other methods of conveying document type

MIME types are not the only way to convey document type information:

  • Filename suffixes are sometimes used, especially on Microsoft Windows. Not all operating systems consider these suffixes meaningful (such as Linux and MacOS), and there is no guarantee they are correct.
  • Magic numbers. The syntax of different formats allows file-type inference by looking at their byte structure. For example, GIF files start with the 47 49 46 38 39 hexadecimal value (GIF89), and PNG files with 89 50 4E 47 (.PNG). Not all file types have magic numbers, so this is not 100% reliable either.

See also