File sharing platform for user contributions? [closed]

I suggest using BuddyPress groups with the BuddyPress Docs plugin (attachments enabled).

You need to allow XML uploads with:

add_filter('mime_types', 'custom_upload_xml');

function custom_upload_xml($mimes) {
    $mimes = array_merge($mimes, array('xml' => 'application/xml'));
    return $mimes;
}