Whitelist of indexed file types

Rather than trying to specify all the file types you don't want indexed, I think it would be easier to specify just the types you do want indexed. Tika is already reasonably limited in what types of files it can parse info from, so even if you wanted to index everything that Tika supports, the list wouldn't be huge. However, if you only have a few types you actually want indexed, then the list of files you don't want indexed is pretty large.