Fix selecting a save filename that ends with the extension does not add an extension

[adamkewley]

Repro:

• Open something that is save-able (e.g. model.osim)
• Then Save As with a filename that is suffixed with the extension (no dot, e.g. someosim)
• The resulting file is called someosim
• But it should be called someosim.osim
• Unless it is suffixed with the dot (e.g. some.osim --> some.osim)