uiimage.c: when creating a new file from the File Manager, require the user to 
enter an extension too [Fabio Priuli]

this matches the behavior from command line where launching files of a
create-able media (e.g. tapes or memory cards) without extension is always 
interpreted like launching from softlist, while launching it with an extension 
actually creates the file if it does not exist yet.

git-svn-id: svn://dspnet.fr/mame/trunk@23292 749742ba-7341-0410-aadc-df50b521781e