These tables are used by the search code to allow search by file type.

Also, I know NATURAL JOIN is used elsewhere in the code (including in this file), but it's a bad idea to use that in production code. Since we know the columns, we should just make them explicit. (Obviously these tables are extremely unlikely to 1) change and 2) change in a way that breaks a natural join, but the syntax is still better thought of as a shortcut for the command line.)