1.Open Nautilus -> Select the file -> File -> Preferences -> Open With -> Select -> Remove."
1.Where you want to go is into your home folder and use ctrl+h or view -> show hidden files.
2.Then open .local -> share -> applications
In the applications folder should be a file named mimeapps.list (it doesn't exist by default but is created from using, among other things, the r.click in certain ways. If it's not there don't worry.
Read this: http://ubuntuforums.org/showthread.php?t=1034632