Imgur bad extension detection

umm, as the title says... [i tried to upload an activity with **jpg**](http://schoolido.lu/activities/2940838/), but it resulted that it reads as `.png` extension instead `.jpg` on the url, so the image won't load.
# Screenshots


Start by reproducing the problem with the linked activity and trace how uploaded Imgur images are assigned extensions in the activity upload flow. Done means a JPG upload is represented with a .jpg URL and loads correctly instead of being reported as .png.

Written by the indexing model from the issue text.