Hello, i've been wondering about this for quite a few years. I used the feature extensively in Dopus 3.0, 4.0 and 5.0, 5.1 and 5.5
It was the possibility to define a custom file type irregardless of the 3-letter extension (on amiga there wasnt any such requirement).
Anyways, i remember how i could define a JPEG file type, by some JFIF header that needed to be present, and then jump to a specified byte delta, and then back and so on, to completely verify that it was a valid file-type.
The argument goes for many other file types, that has inadvertedly been named / typed incorrectly, or viruses, or any other kind of thing that wants to hinder execution or access by the wrong program (e.g. avoiding overflows).
As far as i know, its not currently possible to do this trick anymore. I know that in some cases will use a lot more cpu and a lot more reading of the hd, than a regular 3-letter recognition and linking to a specific file type, but at least i still think it would be a very nice and nifty feature, though, i dont know the reasoning for removing/(not implementing) this feature in the windows versions.
Thank you for any inputs
Best regards : Jacob Eskildsen