ImageProxy: return extension type to user
ImageProxy: return extension type to user

Fixes #16. Reverses a change that was made in relation to https://github.com/FreshRSS/FreshRSS/issues/1257

The underlying cause is something slightly different, see https://github.com/FreshRSS/FreshRSS/issues/1548#issuecomment-304434061
Frenzie/Extensions,FreshRSS/Extensions,FreshRSS/Extensions,Frenzie/Extensions,Frenzie/Extensions,FreshRSS/Extensions