If you try to open a decoded attachment, either by double-clicking or from the context menu, and nothing happens, then no default application is defined for that type of file. First copy the file to somewhere convenient, such as the desktop, then select the file (in Finder) and select the Get Info option. Then under "Open With", select whatever application you'd like to use as a default, and click "Change All". In future, that application will be used to open all files of the type you selected.