A cross-platform answer file?

[Doug Lerner]

In the docs, the example of an answer file for web graphics is:

answer file myPrompt with filter "Web Graphics,*.jpg;*.gif;*.png"

But... this doesn't work for OS X, which seems to require a concatenation of
4-character Mac types.

What are the types in this case? JPEGGIFFPING?

How would one make a cross-platform version of this answer file prompt?

Thanks,

doug