Disabling compression when encrypting

[Robert J. Hansen]

David Shaw wrote:
> Note that GPG also checks the input file to see if it is compressed.  If 
> it sees a compressed input file, it automatically disables compression.

How does it do this?  Does it look for known headers, or does it check 
the entropy of the source text?  If the former, then what headers does 
GnuPG know about?  zip, bz2 and gz, I'm assuming, but what about more 
exotic formats like 7z and the like?