First, save such an attachment to any folder you like. Than use Windows Explorer to find out what the extension is. That's the part of the filename behind the last . (like doc in letter.doc). You might have to set an option to show extensions (not hide them) for known file types in Config Panel>Folder options.
Then, in Config Panel>Folder options>Filetyps search that extension, and find out what program is used to open it by default.