The ExportDialog now calls adjustFileName, which in turns calls
adjustExtension to switch xxx.ps to for instance xxx.png.

adjustExtension removes the extension (everthing after the dot [ps])
and replaces it with the new extension [png], unless the extension
is already a known good extension.

This scheme fails for names such as xxx.heprep.zip which would
be adjusted to xxx.heprep.png rather than xxx.png.

An extra call to the current exportfiletype (before switching)
to remove a known extension and changing behaviour of the default adjustExtension to add the new extension (unless the extension is ok
already) would make the whole thing work.