When trying to import a file type not allowed by the target wiki's $wgFileExtensions, the error dialog looks like:
This is misleading, as we *do* know what the error is, and there's no way to correct it by changing the file's title.
 If a file's file type is not allowed on the target wiki, go straight to the "hard" error page and show the following error message: You're trying to import a file with the extension <extension name>. This file type is not allowed in this wiki.
This is an issue for non-Wikimedia wikis using the file importer. The Wikimedia wikis all have the same file type acceptance criteria.