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A mailcap file can be used to find the correct program to open a file with, based on MIME information. While the name is "mailcap" and is related to MIME (mail filetypes) it actually has use outside the domain of electronic mail (see more about this in MIME).
In UNIX-type systems, the mailcap file is usually located at /etc/mailcap and/or $HOME/.mailcap
RFC 1524 A User Agent Configuration Mechanism For Multimedia Mail Format Information describes the format of the contents of this file.
Lines can be comments starting with the # character, or a mime-type followed by how to handle that mime type. The first part is called the content-type, and the second part is called the view-command.
video/mpeg; xmpeg %s says if a file encoded in mime has type video/mpeg, run the xmpeg program with the file name as a parameter.