FIX: image resizer service not handling certain filenames.

The edge case is a name like "examplejpg.jpg".
The system was failing to put the $ext extension on.
Derivative datastream images like web_ , thumbnail_ and preview_ were not being generated.

Also added an extra check on the final filename that is used.
Merge pull request #20 from danielbush/master-derivative-images

FIX: image resizer service not handling certain filenames.