Internet Explorer uses Multipurpose Internet Mail Extensions (MIME) type information to decide how to handle files that have ...

"Internet Explorer uses Multipurpose Internet Mail Extensions (MIME) type information to decide how to handle files that have been sent by a Web server. For example, when there is a Hypertext Transfer Protocol (HTTP) request for .jpg files, on receipt are generally displayed to the user in an Internet Explorer window. If Internet Explorer receives an executable file, Internet Explorer generally prompted the user for how to handle the file.

In Windows XP Service Pack 2, Internet Explorer follows stricter rules that are designed to reduce the attack surface for spoofing the Internet Explorer MIME-handling logic.

When files are served to the client, Internet Explorer uses the following pieces of information to decide how to handle the file:

- File name extension
- Content-Type from the HTTP header (MIME type)
- Content-Disposition from the HTTP header
- Results of the MIME sniff

In Windows XP Service Pack 2, Internet Explorer requires that all file-type information that is provided by Web servers is consistent. For example, if the MIME type of a file is "text/plain" but the MIME sniff indicates that the file is really an executable file, Internet Explorer renames the file by saving the file in the Internet Explorer cache and changes its extension. (In a MIME sniff, Internet Explorer examines, or sniffs, a file to recognize the bit signatures of certain types of files.)"