thank you very much for giving us some details and background info.
I'm really not interested in the inner workings of your file format. It's perfectly valid for you to keep your file format proprietary so you can change it whenever you need to. I fully understand that.
What I would like to see is a standard/documented way to extract a thumbnail/preview image for .afdesign and .afphoto files. Like it exists for PSD files, DNG files, TIFF files, MP3 files etc.
Or maybe a shell extension for WIndows which allows Windows to pull a preview from your files. This would not only allow us to see previews of .af* files in Windows Explorer (which we currently can't and that's a real nuisance) but also would allow all Windows applications to use the standard Windows shell functions to extract the thumbnail / preview. If you supply such a shell extension with your applications, you don't need to reveal anything about your file formats.
A more modern approach would of course be to provide a WIC codec for your files. WIC is the official approach on Windows to support RAW files and other file formats. Your WIC codec would allow applications to extract a thumbnail/preview without knowing anything about the inner workings of your file formats.
I could not find any information if your software allows users to edit EXIF data or XMP data. I checked in Designer but I could not find any way to input title, description, keywords, author, GPS data etc.
If you already have or plan to add support for XMP soon, it would be very nice to do it in a way that allows other applications to read and write that metadata. Its usually important for private photographers, but a must for pro photographers.
Phil Harvey, the author of ExifTool, is surely willing to support your files in his application and library. ExifTool is the de-facto standard for dealing with metadata on all platforms.
