Items in context menu in Lubuntu

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http://unix.stackexchange.com – I am using Lubuntu 11.10. When I call the context menu (right click) many programs appear to open some file. For example, for .tex files, besides TeXstudio and Leafpad, Gnumeric (what a strange option!) and text editor also appear. So, I'd like to remove those bad options. For some extensions (.txt for example) I could remove the wrong programs by deleting the .desktop files somewhere in .config subfolders. How should I do this? (HowTos)