The OS X versions of tk_get*File (Tk-Cocoa) do not display -filetypes comboboxes/menubuttons when the -filetypes option is specified. The list of possible files is merely filtered. On Linux and Windows a combobox/menubutton is shown and the user can choose a filetype.
On OSX, the setAccessoryView task of the NSSavePanel and NSOpenPanel do support this kind of feature:
It looks as if this is relatively simple to implement and it would bring the OS X file dialogs to the same level as for the other platforms. If no -filetypes are specified, the accessory view could be hidden.
In the future, this accessory view culd even be used to display more options like checkboxes, text and other stuff.