Can I add an additional attribute to the xml declaration (first line) of an xml file?

Question

Is it legal xml syntax to add an additional attribute (to indicate a file-type, for instance) to the xml declaration at the top of an xml file?

(See, e.g., What is on the first line of an XML document?)

Answer 1

It took me a bit of digging to verify that the answer is NO: According to the specification, the xml declaration can only include the xml version, encoding, and standalone document declaration.