Manual:$wgFileExtensions

Details
$wgFileExtensions is the list of allowed/preferred extensions for uploading files in .

i.e. $wgFileExtensions is then only a preferred set of extensions.

In this case some particularly dangerous file extensions will still be blocked using $wgFileBlacklist and $wgMimeTypeBlacklist.

If  is false, this option has no effect since all file extensions will be permitted without performing any checks.

If  is set to false, this option has no effect since all uploading is disallowed.

Some files, for example OpenDocument files (e.g. used by OpenOffice.org as odt, ods, odg, ...), use MIME types that don't match with their extension (or something similar).

Uploading such files results in an error message like.

In such cases you can adjust the mapping of file extensions to mime types, see man>Special:MyLanguage/Manual:Mime type detection#MIME type validation|MIME type validation.

Example
$wgFileExtensions is a php array, which is initialised with a default (sensible) set of file extensions.

In LocalSettings.php you might use array operations such as...