Even if it's varint separated messages?

You stick a magic number and a version number at the top of the file and call it done. It’s trivial and buys you a great deal.

The magic means it’s possible to identify the file type. Maybe you’ll add a tool later that operates on multiple types of files.

The version means you can evolve the contents in non-backwards-compatible ways, while maintaining the ability to read/parse the old version.

It’s a pain in the ass to add a magic or version number later; there’s a reason why nearly every file format on the planet has both.

> nearly every file format on the planet

XML

JSON

YML

TOML

  <?xml version="1.1" encoding="UTF-8" ?>

The others are serialization formats, like protobuf — they’re not file formats.

XML prolog is optional.

> they're not file formats

Brb, gonna delete all my files without file formats

If you prefer to ship fragile binary file formats for no reason other than finding it too onerous to define something as trivial as an eight-byte file header, that’s silly, but nobody is going to stop you.

If you’re going to use text-based serialization formats as your justification for the decision, however, I’d suggest you look into all the fun bugs, security issues, and weird edges cases that arise from parsers having to make a best guess at character encoding and file format when all you have to work with is the file extension, maybe a byte order mark, and heuristics over the file contents.