[BUGFIX] GIFBUILDER files cannot be delivered via web server

GIFBUILDER uses provided file names and text snippets in order to
generate the final name for files to be written to. In case text
snippets start with a dot, this is also reflected into the file
name - preventing some web servers to deliver the file since it's
considered to be internal.

Resolves: #83029
Releases: master, 8.7
Change-Id: Iebed832c1c2429c7400373a6af4d5ae14a9ea18e
Reviewed-on: https://review.typo3.org/54677


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>

typo3/sysext/frontend/Classes/Imaging/GifBuilder.php

@@ -699,7 +699,7 @@ class GifBuilder extends GraphicalFunctions
         /** @var $basicFileFunctions \TYPO3\CMS\Core\Utility\File\BasicFileUtility */
         $basicFileFunctions = GeneralUtility::makeInstance(BasicFileUtility::class);
         $filePrefix = implode('_', array_merge($this->combinedTextStrings, $this->combinedFileNames));
-        $filePrefix = $basicFileFunctions->cleanFileName($filePrefix);
+        $filePrefix = $basicFileFunctions->cleanFileName(ltrim($filePrefix, '.'));
 
         // shorten prefix to avoid overly long file names
         $filePrefix = substr($filePrefix, 0, 100);