[CLEANUP] Fix docheader param for Folder:addFile()

Add missing conflictMode 'changeName'.

Releases: 6.2, 6.1, 6.0
Resolves: #57004
Change-Id: Ie5c32e96c1292dbaecc505d54240f1a7a5c720fe
Reviewed-on: https://review.typo3.org/28476
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer