MIME

MIME stands for Multi-Purpose Internet Mail Extensions. In its original version, it defined certain data types of email attachments so that these files could be read and executed when they were not in ASCII format. MIME specification include various multimedia formats such as image files, video, text, and numerous application formats.

General information

MIME has proved particularly useful in dealing with data types and is no longer just used as a reference to the format of an e-mail attachment. MIME specifications have been established as a standard in the field of Internet protocols. The reason is that specific metadata is transferred with every dialogue between a client and a server. MIME defines an important part of this metadata which is located in the header of an HTML document. Both the server and the browser will then know what type of data to transmit and just read or execute.

MIME types are specified as standard by IANA, which is an organization that is dedicated to the allocation of IP addresses and other Internet standardization issues.

Practical relevance

If you open an email which contains images, these images can be directly displayed thanks to the MIME types. Because if the data type is defined, for example, with the GIF format for images, the e-mail program will be able to know how such files are to be handled. The principle is similar to a dialogue between client and server. Servers transfer MIME data at the beginning of each session. Browsers get from the MIME data in the header what application is to be used for the data type in question. Some applications are already integrated into the browser, but others must be loaded externally. If there is no application to run the file, the file is usually simply downloaded for later use or a recommendation is given for an appropriate program.

HTML documents also contain elements that identify a MIME data type as the value of an attribute. Examples include links, scripts, and parameters. A data type can be determined with the attribute “type.” It is also possible to define your own data types with MIME, if these do not already exist in the IANA standard.

The scheme of MIME types is defined as part of Requests for Comments (RFCs) of the Internet Engineering Task Force (IETF). Each MIME type consists of the specification of a media type and a subtype. A slash separates the two types of information. For example: text/html, image/gif or application/mspowerpoint. Subtypes for server-side file formats are introduced with the preamble “x.” The following media types exist:

• text = for text files

• image = for graphics files

• video = for video files

• audio = for sound files

• application = for program-dependent files

• multipart = for multipart data

• message = for messages

• model = for complex files

Relevance to SEO

The algorithms of search engines are always trying to read the entire contents of an HTML document. A few years ago they could not yet crawl file formats such as images, videos or podcasts. Although search engines cannot read the entire contents of the MIME data types, they at least know what types of data can be found in the document in question and thus have an indication as to the content. Furthermore, additional metadata can be passed on to the user agent of a search engine, so it is clear how the content is classified.

Currently, all search engines are working to make more data types crawlable. PDF documents or PowerPoint presentations are now no longer a problem. If you want to load certain types of data on the server or the client, it is recommended from an SEO perspective to pass the respective MIME type with the header of the document.

Web Links

• What are MIME types?
• Definition of MIME data types