CFZIP problems - Unsupported File Format

[I tried researching this via Google, but the terms are so generic I didn't find any relevant hits]

I am working with files that originate via incoming emails as attachments.  In CF I detach them from the emails and hand them off to other jobs for processing. Sometimes they are zipped files, and I have been using CFZIP to extract the actual data file and process it.

I started getting new files from a client and they are zipped (.ZIP file type), and if I open them from Outlook WinZip opens them just fine.  If I open them in Notepad, they all start with "PK" which I have always believed to mean that they were zip (PKZIP) format, as opposed to other flavors such as GZip.  But when the CF script tries to open that same file with CFZIP, it throws an Unsupported File Format error.

Any thoughts of diagnosing this error, to determine if it really is a valid ZIP format file?  Are there any utilities out there that would give me info on exactly what program/algorithm was used for the compression process? 

In the past when I've had these problems it was because the file was .GZ (Gzip format) and not .ZIP, and in those cases even Winzip indicated as much when it opened the file.  But this is perplexing because Winzip seems to think the file is just fine, and CF doesn't.

Thanks for any ideas!

-reed