Hi!
Is Justin's suggestion of aliases feasible, so that "filetype:image" is the same as "filetype:bitmap"? there's still the question of what to map to what, but that seems like it would help if it's easy enough to implement. And of course a histogram of filetype:<x> would tell you what people are trying to use.
But maybe adding aliases to reserve words is too messy.
Created https://phabricator.wikimedia.org/T150887 for it. Using logs to see what people are using with filetype: is a great idea too, we'd have to wait for a month or so to gather data I assume.