[FEATURE] Allow ext_icon.png as extension icon besides ext_icon.gif

Follow-Up to #37595

Fixes two things found during the review. Due to an unfortunate
timely intersection the original patch set was merged without
these fixes.

Change-Id: Ic9c69f09c1e92c628cd29c5c303e1807022a2a8e
Fixes: #37595
Releases: 6.0
Reviewed-on: http://review.typo3.org/13898
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Ingo Renner
Tested-by: Ingo Renner