MIME 類別 (IANA 媒體類別)

最簡單的 MIME 類別由*主類別(type)和子類別(subtype)*組成。兩個都是字串，並由一個斜線(/)相接，且不能包含任何空格：

type/subtype

主類別(type) 代表廣泛性的分類，譬如video 或 text。子類別(subtype) 則定義該資料精確的 MIME 類別。例如 MIME 類別為text, 子類可能會是 plain (純文字), html (HTML 源碼), 或calendar (iCalendar/.ics) 檔案。

每一種主類別都可能有一組自己的子類別，MIME 類別永遠都有主類別和子類別，從來不會只有單一個。

後面加上的參數可以提供更多細節：

type/subtype;parameter=value

例如一個 MIME 類別的主類是 text, 選擇性的 charset 參數可以用來指明資料種所使用的字元集。如果沒有指明charset ，預設使用 ASCII (US-ASCII) ，除非被user agent's (en-US) 的設定覆蓋過去。 要指明一個 UTF-8 的文字檔，可以使用 text/plain;charset=UTF-8 的 MIME 類別。

MIME 類別對大小寫不敏感，但通常都會使用小寫。

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 (en-US) 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 (en-US) header).

這是二進制檔案的預設類別，代表未知的二進制檔案，通常瀏覽器都不執行或是會詢問是否要執行。They treat it as if the Content-Disposition (en-US) header was set to attachment, and propose a "Save As" dialog.

文字檔案的預設類別。就算是未知的文字檔案，瀏覽器都先假設他們是可以被呈現於畫面的。

用來套用在網頁的 CSS 檔案一定要配合 text/css 做傳輸。如果伺服器沒有將副檔名 .css 視為 CSS 檔案，伺服器有可能會使用 text/plain 或是 application/octet-stream 的 MIME 型態來傳輸檔案，而導致這些檔案不被大多數的瀏覽器當成 CSS 而被忽略。

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

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

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.

Files whose MIME type is image contain image data. The subtype specifies which specific image file format the data represents.

The following image types are used commonly enough to be considered safe for use on web pages:

• image/apng (en-US): Animated Portable Network Graphics (APNG)
• image/avif (en-US) : AV1 Image File Format (AVIF)
• image/gif (en-US): Graphics Interchange Format (GIF)
• image/jpeg (en-US): Joint Photographic Expert Group image (JPEG)
• image/png (en-US): Portable Network Graphics (PNG)
• image/svg+xml (en-US): Scalable Vector Graphics (SVG)
• image/webp (en-US): Web Picture format (WEBP)

The Image file type and format guide (en-US) provides information and recommendations about when to use the different image formats.

As is the case for images, HTML doesn't mandate that web browsers support any specific file and codec types for the <audio> (en-US) and <video> (en-US) elements, so it's important to consider your target audience and the range of browsers (and versions of those browsers) they may be using when choosing the file type and codecs to use for media.

Our media container formats guide provides a list of the file types that are commonly supported by web browsers, including information about what their special use cases may be, any drawbacks they have, and compatibility information, along with other details.

The audio codec (en-US) and video codec (en-US) guides list the various codecs that web browsers often support, providing compatibility details along with technical information such as how many audio channels they support, what sort of compression is used, and what bit rates and so forth they're useful at. The codecs used by WebRTC (en-US) guide expands upon this by specifically covering the codecs supported by the major web browsers, so you can choose the codecs that best cover the range of browsers you wish to support.

As for MIME types of audio or video files, they typically specify the container format (file type). The optional codecs parameter (en-US) can be added to the MIME type to further specify which codecs to use and what options were used to encode the media, such as codec profile, level, or other such information.

The most commonly used MIME types used for web content are listed below. This isn't a complete list of all the types that may be available, however. See the media container formats guide for that.

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 (en-US), 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--

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 (en-US) 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--

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> (en-US) or <audio> (en-US) elements. Be sure to specify the correct media 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.

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 (en-US) header.

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.

• Web media technologies
• Guide to media types used on the web
• Properly configuring server MIME types (en-US)