To me, it looks like overriding SpecialPage::outputHeader would be the right thing to do in this case.

Well, what you want here isn't to tell the directionality of the content language, but to tell the inherent directionality of the File namespace's default name in the current content language, I think?

You don't necessarily know whether it's going to be localized (as it is in Hebrew) or an English fallback name.

And honestly since it's written in an English context here, it looks reasonably ok to have the colon appear after, as in English.

The best fix would actually be to just get rid of the message entirely and make sure that Special:Filepath takes input with or without the namespace name -- it's simply a totally unnecessary requirement and there's no reason to bother the user with the demand in the first place.

Ok, looks like the 'without the namespace' limitation was a regression in r43471 -- prior to that, Special:Filepath handled input using Title::newFromText($file, NS_IMAGE) which would work both with and without the namespace.

Any objection to reverting r43471 and removing the extra line from filepath-summary?

Done in r97558 on trunk.

Merged to 1.18 as r97559.

Note that translated messages will still need updating -- should happen as normal through translatewiki.net.