Favicon: Select Favicon appears when a non image file has been selected

[zcolah]

Bug: Right now when a user selects a a non-image asset as a favicon they get an error similar to this.

Solution Only show image files in the options available to select as a favicon in the media selector.

[shrunyan]

Of note is the favicon test is failing at the moment

[shrunyan]

related https://github.com/zesty-io/manager-ui/issues/1463