Ah, sorry, didn't realize. I've done some routing around and found this:http://ubuntuforums.org/showthread.php?p=2161195#8.
With all of the custom one's I've made they appeared as items in
and from what small tests I've done removing the relevant items there removed them in my open with list.
There are two mime files in mine that I have left well alone, and if you don't recognize an item in the list my advice is to also leave it well alone, or make a copy of it somewhere else before removing it.
Hope that helps,