Manual:$wgFileExtensions

Details
Example:

This is the list of preferred extensions for uploading files. Uploading files with extensions not in this list will trigger a warning when $wgStrictFileExtensions is false. However $wgStrictFileExtensions defaults to true, blocking any file extensions not in this array. This option does not have any effect if $wgCheckFileExtensions is false. Some filetypes are on a blacklist: $wgFileBlacklist.

See also: Manual:Mime type detection