[BUGFIX] Cannot upload an extension as zip

The file extension ".zip" should be systematically removed when
extracting the extension key from the uploaded file name.

Fixes: #52178
Releases: 6.2, 6.1, 6.0
Change-Id: I5316b995533fa324d3b19407ad9bb6365ab8858b
Reviewed-on: https://review.typo3.org/24037
Reviewed-by: Sebastian Fischer
Tested-by: Sebastian Fischer
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn