https://github.com/wikimedia/pywikibot-core/blob/d616db5/scripts/checkimages.py#L1480 defined notallowed as ("xcf", "xls", "sxw", "sxi", "sxc", "sxd").
Instead of using this hard coded list, the list of allowed extensions provided by the server should be used.
See https://commons.wikimedia.org/w/api.php?action=query&meta=siteinfo&siprop=fileextensions for the server provided list.
See b5b79ec57 for a similar improvement that was recently made to upload.py.
The last time this list was modified was in 2008 1bc7ef19 , removing djvu from the list when djvu support was added to Wikimedia wikis.
Note change https://gerrit.wikimedia.org/r/#/c/256387/ has a related modification, removing "xcf" from the list.