[brabel]

> They had a hard limit that they wouldn't see past the first 256 bytes.

Then they could never detect zip files with certainty, given that to do that you need to read up to 65KB (+ 22) at the END of the file. The reason is that the zip archive format allows "gargabe" bytes both in the beginning of the file and in between local file headers.... and it's actually not uncommon to prepend a program that self-extracts the archive, for example. The only way to know if a file is a valid zip archive is to look for the End of Central Directory Entry, which is always at the end of the file AND allows for a comment of unknown length at the end (and as the comment length field takes 2 bytes, the comment can be up to 65K long).

[jeffbee]

That's why the whole question is ill formed. A file does not have exactly one type. It may be a valid input in various contexts. A zip archive may also very well be something else.