TYPO3 Core

The problem seems to come from here: https://github.com/TYPO3/typo3/blob/e3f666cd9c1157bb27edda363807ab05f047bf20/typo3/sysext/extbase/Classes/Mvc/Web/RequestBuilder.php#L131-L145

Line 143 breaks the array if you have only one file input. I suspect it was introduced to undo line 135. But that one will not be executed normally. If we have an UploadedFile instance on line 135, wouldn't that come from a file input with name="tx_myext_myplugin". That would be odd. I would argue, that such a input parameter shouldn't make it into the extbase request object in the first place. So I would change line 137 to $files = []; and remove the lines 142 - 144.

This was fixed in main already, see #102271. Should still be fixed for v12 IMO.

During a session at T3CMD24 we realized that this behaviour actually comes from the backwards compatibility layer introduced in #97214; that layer was removed for main, but in v12, some people might rely on it now since v12 was released quite a while ago. Therefore, I personally would not fix it anymore to prevent breaking instances with a patchlevel release.