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mbin most likely refers to "MacBinary" (a format used to preserve the Mac resource fork when a file is passed around online/on non-Mac file systems)

Indeed, I use .mbin for MacBinary, as the more conventional .bin is overloaded with other meanings.

I also use .mbim for MacBinary+, which encodes a directory tree instead of a single file.

I've tried different versions of Macbinary without luck, maybe I'm missing something.