The TYPO3 extension "FAL File Cleanup" offers an easy way to identify and automatically clean up unused files in the TYPO3 system. 
A TYPO3 scheduler task carries out regular checks in the system and compares the linked files (file references) with the existing files in the TYPO3 file admin. If desired, these regular checks can also be limited to certain file endings.
If files are identified which do not have a link in the TYPO3 system content, they can be deleted directly, moved to a recycle bin first and deleted after definable intervals (e.g. after 30 days).