Manual:$wgStrictFileExtensions

Details
If set to true, users will only be able to upload files with specifically permitted extensions (see  ).

If set to false, users will be able to upload files with almost any extension.

A user attempting to upload a file whose extension is not in   will receive a warning message.

However, they may choose to ignore the message and upload the file anyway (assuming the file's extension is not in the  ).