Conditional IMAGETYPE possible w/ CGI?

[Dave Fuhry]

I'm using CGI mapserver, and would like it to return JPEGs when an
ortho layer is active, and 8-bit PNGs otherwise.

The CGI reference makes no reference of an exposed IMAGETYPE or
equivalent parameter.  Variable substitution in the mapfile (IMAGETYPE
"%param%") doesn't seem to work.

Are there any easy ways to conditionally specify the IMAGETYPE using
CGI mapserver, or will I have to preprocess the mapfile?

Thanks,

Dave Fuhry