The distinction between file format and payload format is perfectly normal and not at all useless. All file formats have limits for how much they define types

The distinction between file format and payload format is perfectly normal and not at all useless. All file formats have limits for how much they define types for the payload. Most have very limited types. JSON for example knows objects, arrays, strings, numbers, booleans and null. It's not that big a difference to have a format that has only strings.