#3 msworkbench fails with image type PNG, JPG

[Tom Poindexter]

per Matt Wilkie via mapserer-users list:

> The App Wizard sometimes works and sometimes doesn't. It is
> more likely to not work when the image type is PNG. It is
> more likely to work when the image type is GIF, and it is a
> totally new map (eg: new directory with no contents).

tp: need to only offer the same image types
that mapserver is compiled with.