The TCP/IP Guide - MIME Content-Type Header and Discrete Media: Types, Subtypes and Parameters

This media type is a “catch all” for any kind of data that doesn't fit into one of the categories above, or that is inherently application-specific. The subtype describes the data by indicating the kind of application that uses it. This can be used to guide the recipient's e-mail program in choosing an appropriate application program to display it, just like how a file extension in Windows tells the operating system how to open different kinds of files.

For example, if you have Microsoft Excel installed on your PC, clicking a file ending with “.XLS” will launch Excel automatically. Similarly, an Excel spreadsheet will normally be sent using MIME with a media type of application/vnd.ms-excel. This tells the recipient's e-mail program to launch Excel to read this file.

Since there are so many applications out there, there are over a hundred different subtypes within this top-level type. Table 247 contains a few representative samples.

**Table 247: MIME *application* Media Type Subtypes**
type/subtype	Description	Defining Source
*application/octet-stream*	An arbitrary set of binary data octets. See below for more details.	RFC 2046
*application/postscript*	A Postscript file, used for printing and for generating Adobe Acrobat (PDF) files.	RFC 2046
*application/applefile*	Resource file information for representing Apple Macintosh files.	Registration with IANA
*application/msword*	Microsoft Word document. Note that this does not have the “vnd” prefix like most other Microsoft file types.	Registration with IANA
*application/pdf*	A Portable Document Format (PDF) file, as created by Adobe Acrobat.	Registration with IANA
*application/vnd.framemaker*	An Adobe FrameMaker file.	Registration with IANA
*application/vnd.lotus-1-2-3*	A Lotus 1-2-3 file.	Registration with IANA
*application/vnd.lotus-notes*	A Lotus Notes file.	Registration with IANA
*application/vnd.ms-excel*	A Microsoft Excel spreadsheet file.	Registration with IANA
*application/vnd.ms-powerpoint*	A Microsoft Powerpoint presentation file.	Registration with IANA
*application/vnd.ms-project*	A Microsoft Project file.	Registration with IANA
*application/zip*	A compressed archive file containing one or more other files, using the ZIP/PKZIP compression format.	Registration with IANA

Of these, there is one special subtype that is worth further mention: the application/octet-stream subtype. This is a “catch all” within the “catch all” of the application type, and just means the file is a sequence of arbitrary binary data. It is usually used when the sender is unsure of what form the data takes, or cannot identify it as belonging to a particular application. When this type is used, the recipient will usually be prompted to just save the data to a file. He or she must then figure out what application to use to read it.

The application/octet-stream MIME type/subtype may even be used for images, audio or video in unknown formats. If you try to send a multimedia document that your sending program does not understand, it will generally encode it as application/octet-stream for transmission. This is your e-mail program's way of saying to the recipient “I am sending you this file as-is, you figure out what to do with it”.

This application/octet-stream type is also very often used for transmitting executable files (programs) especially on Windows systems. Unfortunately, while convenient, this can be a serious security hazard. In recent years, the Internet has been subject to a steady stream of viruses and worms that spread by sending themselves to other users through executable file attachments in e-mail. This makes opening and running any unknown application/octet-stream attachment potentially dangerous.