Issue #50: Keep existing file extension
Issue #50: Keep existing file extension

This just pulls the extension from the original filename.
It'll fall apart when any of the following are true:
- Filename is missing extension
- Extension separator is not "."
- Extension is not supported by conversion program

TODO: Fall back to JPG if extension isn't one of the
supported types (gif, jpg, png)

briankassouf/mandible,henrylee2cn/mandible,josephyzhou/mandible,Imgur/mandible,josephyzhou/mandible,henrylee2cn/mandible,briankassouf/mandible,Klaasvaak/mandible,Klaasvaak/mandible,Imgur/mandible