If the argument to make_file can act like a file, return it. This is useful for code that calls command functions, but doesn't want to send I/O through the filesystem.

header = header-rss.tmpl
changelog = changelog-rss.tmpl
changelogentry = changelogentry-rss.tmpl
filelog = filelog-rss.tmpl
filelogentry = filelogentry-rss.tmpl