To get both, try this:
GetFileOpenName ( "Open File", pathname, filename, "TXT", "Graphic Files (*.bmp;*.gif;*.jpg;*.jpeg),*.bmp;*.gif;*.jpg;*.jpeg,All Types(*.*),*.*" )
You'll see two options in the dropdown, and the first has multiple file types. Note semicolons between the filetypes and commas between the sets of options (as well as between the description and the actual filter value).