TYPO3 Core

I can confirm the whole thing for the v11/current master.

You can still secure the code quite well and return a "success: false". The tests then do not throw any errors for the time being.

The question is? What does one expect here as a user?
- Should the GIF tests be removed? It doesn't make sense at first, because you still want to convert. (See .tif example)
- We could output a "Please support gif" flash message, but that might just confuse the integrator. (And is actually not the user's wish either)
- We could manipulate the tests. So they are running always with GIF. Independent of the imagefile_ext setting (My favorite)

The trigger is that the function getImageDimensions() in typo3/sysext/core/Classes/Imaging/GraphicalFunctions.php checks for $this->imageFileExt. gif is now missing there and null is returned.
https://github.com/TYPO3/TYPO3.CMS/blob/master/typo3/sysext/core/Classes/Imaging/GraphicalFunctions.php#L2193

With a public setter we can force the gif extension from the EnvironmentController. Here a simple and hacky example ;-)

$fileExt = explode(',', $GLOBALS['TYPO3_CONF_VARS']['GFX']['imagefile_ext'] . ',gif');
$imageProcessor->setImageFileExt($fileExt);

Hacky does not sound promising. OTOH, dependencies to globals aren't great either.

Within the installer it may not be of interest what files may be uploaded by a user.

If the instance can handle GIFs used in TypoScript or Fluid directly, then the installer must be able run tests on them.

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/67344

Patch set 3 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/67344

Patch set 4 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/67344

Patch set 5 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/67344

Patch set 1 for branch 10.4 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/c/Packages/TYPO3.CMS/+/67305