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But if there were "no extension", that would be a 0-length extension, rather than a 3 character extension with 3 spaces.

Even if it has to be 3 characters, shouldn't they at least be \0?

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Since spaces are in fact not allowed, choosing between 0 and 0x20 is pretty arbitrary here. You’d probably go for 0 today because C became popular and 0-padding has the useful property of also 0-terminating the string, but at the time that was less of a consideration.

I assume any extension that consists solely of any combination of \x00 and \x20 is to be understood as "no extension"

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