Prevent the use of non-image files as a background image - #2206

Please follow these steps before submitting a new Pull Request to Guake:

• rebase on latest HEAD:

  $ git pull --rebase upstream master

• hack your change

• to execute the code styling, checks and unit tests:

  $ make style check reno-lint test

• describe your change in a slug file for automatic release note
  generation, using:

  $ make reno SLUG=<short_name_of_my_feature>

  and edit the created file in releasenotes/notes/.
  You can see how reno works using pipenv run reno --help.

  Please use a generic slug (eg, for translation update,
  use translation, for bugfix use bugfix,...)

• create new commit message

  $ <hack the code>
  $ git commit --all

• If your change is related to a GitHub issue, you can add a reference
  using #123 where 123 is the ID of the issue.
  You can use closes #123 to have GitHub automatically close the issue
  when your contribution get merged

• Semantic commit is supported (and recommended). Add one of the following
  line in your commit messages:

  # For a bug fix, uses:
  sem-ver: bugfix
  
  # For a new feature, uses:
  sem-ver: feature
  
  # Please do not use the 'breaking change' syntax (`sem-ver: api-break`),
  # it is reserved for really big reworks
  

Fixes #2205