[Mikhail_Edoshin]

In old days Mac OS files had two properties: file type and creator type. File type could be like "TEXT" and creator type was a unique code for the application that created it (they were registered at Apple, I believe). All this was purely internal, users never saw such things: Mac file names normally didn't have an "extension", it was just "1995 Report" or something.

As a result it behaved like that:

- When you opened a file, it opened in the program that created it.

- When there were no such program, you could still open it in another program that claimed to understand these files. I don't remember how it was implemented, I think there was a dialog asking if one would like to use another program with some choice (probably the first one that fit).

This was rather handy with formats like "Encapsulated Postscript" in desktop publishing: this format supported both vector and raster images, but one normally used different applications to manipulate them.

This stayed for some time after the move to Mac OS X, but now I believe these things are gone or not used at all.