I don’t know anything about this, but the files may be encrypted blobs, but if they are mapped to the original filenames (as is the case with Dropbox) with suffix like jpg, etc, they could assume the type without decoding the file. Not saying there’s no difference between Dropbox and Apple, but I’m not sure people expected filenames to be encrypted back in the day (if even now).
Yeah, to be clear, what the friend was saying that day is that they don’t even have access to file names. For them it’s 100% mangled data.
I would definitely consider file names to be personal information, that I would expect to be encrypted. If I store a file named “Letter to IRS for 2020 violation.doc”, then suddenly you know something about me that I probably don’t want you to know.