text: Title: Set video to main video in edit screen throws: 'File' object has no attribute 'content_type'
  Body: ## Steps to Reproduce
  
  <!-- Please include as many steps to reproduce so that we can replicate the problem. -->
  
  1. Upload a video without setting it as main video
  2. Edit the uploaded video
  3. set it as main video
  4. get this error message
  
  
  whole error message in html:
  [AttributeError at _de_exercise_video_677_1_edit.html.txt](https://github.com/wger-project/wger/files/8497979/AttributeError.at._de_exercise_video_677_1_edit.html.txt)
  
  PS: Aren't this too much sensible informations for a productivity error page?
  
  EDIT: I don't choose an video for replacing the old one.