Fix for wrong mime type mismatch detection

[pawelfloryan]

Since when dealing with multiple extensions related to a single mime type there are multiple rows for each extension, the query will flag a mismatch if the extension was different from the first extension found for the mime type in the valid_mime_types table, even though the same extension for that mime type could be in some other row.

This approach doesn't change the db schema and instead works on a query which separates aggregated extensions between the mime types.