Manual:$wgFileExtensions

Details
Example:

//Add one filetype $wgFileExtensions[] = 'pdf'; //Override the default with a bundle of filetypes: $wgFileExtensions = array( 'png', 'gif', 'jpg', 'jpeg', 'ppt', 'pdf', 'doc', 'mp3','xls', 'zip');

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