Trying to upload a .gif results in 500

[lislis]

We could handle that case properly

 CarrierWave::ProcessingError occurred in users#update:

  Failed to manipulate with MiniMagick, maybe it is not an image? Original Error: `identify /app/tmp/1511218084-6-0001-9806/medium/4LISA.jpg` failed with error:
identify.im6: insufficient image data in file `/app/tmp/1511218084-6-0001-9806/medium/4LISA.jpg' @ error/jpeg.c/ReadJPEGImage/1008.

  app/controllers/users_controller.rb:70:in `block in update'

[lislis]

I added a mention of supporting file types in #218
But I'd like to use something like https://github.com/blackwinter/ruby-filemagic and give real feedback

[lislis]

possibly related to #255 as gifs can be quite large. needs investigation