If an article has a non image file attachment, e.g. xls or PDF, then the forum view article preview just shows a grey blank area as if it's trying to show an image but obviously, it can't.
(I'd like to have tested this here first - can we have a test article forum please?)
Thank you for reporting this issue, it has now been resolved. We are aiming to include any changes that have been made in a future XF release (2.2.0 Beta 5).
Change log:
Ensure only images are used as article cover images.