It just depends on the way the file is actually is in practice.

For xslx you basically get a XML file with a single tag, which then contains the whole content as a binary blob, stored in an attribute.

Just like you can make an svt that is essentially a single path tag with hundreds of numbers in a single attribute

And for the record, you can also do a single html tag with a gigantic inline blob binary. Calling that source code is just at best technically correct, because it practically isn't