Fix for EXT-3730 (Current (temporary) Find URL returns an HTTP status code of...

Fix for EXT-3730 (Current (temporary) Find URL returns an HTTP status code of 500 (Internal server error))

We now no longer check the error code returned from the MIME type probe.
If we have a mime type, use it, regardless of the status.  If not, default to the web plugin and let it handle error reporting.