MIME types allow the classication of electronic file types.
MIME -- Multipurpose Internet Mail Extensions -- is a standard codified by the IETF, the Internet Engineering Task Force. The purpose of MIME was to provide a way to identify file types that were being sent by email over the internet. Over the years other internet applications, such as web browsers, have also come to utilize the MIME standard.
Purpose of MIME
MIME was developed when email clients started to send files as attachments. By defining what type of file was attached, the receiving email program could look for a corresponding application to open the file. This is why, for example, MS Word opens when you receive a .doc file as an attachment. This has been expanded further so that both email programs and browsers can often open files inside their own windows. In addition, MIME types can be used for security; particular MIME types can be banned, moved to quarantine or checked to make sure they contain no hidden problems.
The MIME Naming Standard
A MIME type comes in two parts: a type and a subtype. Common types are application, audio and image. The mime type for MS Word is application/msword. This indicates that it is in application file (to be opened by an application) and that it is specifically for MS word. It is up to the receiving email or browser to provide an association with the program; most browsers and email programs come with this list built in, including the ability to add or modify which program to use. As an example, audio files can often be played by many different programs; these programs can modify the local list of MIME types to make themselves the default, if that is desire of the user.
MIME Subtype Parameters
Subtypes can have optional parameters sent with them. The subtype of "text" will allow a parameter of "charset" to be sent. There are many character sets for text files; by specifying the character set, the receiving application knows which of these to use. For example, HTML, the formatted text of a web page, normally uses the UTF-8 character set; this MIME type is sent as "text/html; charset=UTF-8."
Custom MIME Types
It is possible to create custom MIME types. When building a web application there is often a need to trigger an event by sending a file through email, via a web browser or by retrieving it with a program. Creating a MIME type -- or several MIME types -- to handle this is simply a matter of defining the file and adding the MIME type to the list in the email and browser programs handling it (or to the list on the web server running the receiving program). Most web programming languages, such as Java and PHP, provide ways to detect MIME types, and this information can then be used to decide process the file.
