Asset manager - if you edit a non-image file it breaks

[dannyuk1982]

Expected behaviour

Editing a non-image file (i.e. a PDF) either should not have an edit button, or should not break

Actual behaviour

A popup with "editing asset" appears with a loading graphic that never goes. The following is logged:

[2016-11-15 17:29:46] production.ERROR: Exception: Image [blah.pdf] does not actually appear to be an image. in /home/public_html/statamic/core/Imaging/ImageGenerator.php:178 Stack trace: #0 /home/public_html/statamic/core/Imaging/ImageGenerator.php(130): Statamic\Imaging\ImageGenerator->validateImage() #1 

Steps to reproduce

1. Open Asset manager
2. Click the edit pencil button on something like a PDF