Adding this code to LocalSettings.php will allow you to upload different filetypes to your wiki.

$wgFileExtensions = array_merge( $wgFileExtensions, array( 'pdf', 'ppt', 'doc', 'docx', 'xls', 'xlsx' ) );

To allow uploads you also need to change the owner of the folder to www-data.

# type in shell
chown -R www-data /var/www/wiki/images/

References:
mediawiki.org/wiki/Manual:$wgFileExtensions
mediawiki.org/wiki/…/Trouble_uploading_after_installation

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