In a module debug console on Commons
= mw.title.new( "File:IMAGE.jpg" ).isRedirect returns "true"
In a module debug console on English Wikipedia
= mw.title.new( "File:IMAGE.jpg" ).isRedirect returns "false"
the file is a redirect and users outside of Commons need to be able to test for it.
The issue come up when users from outside Commons are trying to delete file redirects on Commons because they are undetectable and are breaking their templates. See here.
It seems to be a problem with how file redirects in Commons are accessed in other wikis. For example, if we try to access File:冰-bronze.svg on Wiktionary, we immediately go to File:冰-bronze-warring.svg, without any indication that it was redirected.