[BUGFIX] EXT:documentation does not support ext_icon.png

Since TYPO3 6.0, extensions may provide an icon as png instead of gif.

Fixes: #52667
Releases: 6.2
Change-Id: I93eb8790f9f5977b0ac23394f753711450a5ec98
Reviewed-on: https://review.typo3.org/24533
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn