The Download dialog allows a use to choose the program with the filepicker when the "open using an application" option is checked. However many users will just want to specify a program that is in their program search path here, without worrying where the program is located. There should be a way to in addition to picking the binary also enter a command. A similar possibility should also exist in the corresponding "Advanced" dialog for handling file types. There should also be some indication on how to pass the file(name) and arguments to that program.

OK, DUPing my own bug: bug 56662

Thanks Johann ! *** This bug has been marked as a duplicate of 56662 ***