A .mim or .mme file is a file in the Multipurpose Internet Mail Extension (MIME) format. MIME is a specification for the format of non-text e-mail attachments that allows the attachment to be sent over the Internet. MIME allows your mail client or Web browser to send and receive things like spreadsheets and audio, video and graphics files via Internet mail.
MIME was defined in 1992 by the Internet Engineering Task Force (IETF). The distinguishing characteristic of a MIME message is the presence of the MIME headers. As long as your mail recipients also have e-mail software that is MIME-compliant (and most e-mail software is), you can swap files containing attachments automatically.
Here are some tips for e-mail attachments:

• You can use a utility like WinZip (PC) or StuffIt (Mac) to compress a large file before you send. Most e-mail systems will not accept messages that exceed a certain size.
• It is a good idea to send attached photographs in the Joint Photographic Experts Group (JPEG) format rather than the GIF or TIFF formats. JPEG (.jpg) files use less space and therefore upload faster.
• AOL and some e-mail software uses .mme or .mim files as a "wrapper" for mail that contains non-text attachments. Use care when sending a MIME attachment to users of older versions of AOL software, as the earlier versions do not handle MIME very well. Send one attachment per e-mail. If you are an AOL user, use the keyword (Ctrl+K) MIME to view help about decoding MIME files.