[ticket/11145] Wrong error thrown when uploading a bigger image.

When attaching an image to a post with a size larger than maximum defined in
php.ini, 'ATTACHED_IMAGE_NOT_IMAGE' error is thrown. $file->is_image() is
returning false value when image size is too large. Therefore, moved
not image test down after image size test.

PHPBB3-11145
Merge remote-tracking branch 'erangamapa/ticket/11145' into develop-olympus

* erangamapa/ticket/11145:
  [ticket/11145] Wrong error thrown when uploading a bigger image.