Considering the definition of a binary file as "a computer file that is not a text file", they can differentiated by searching for the text/plain MIME in their MIME hierarchy.

text_vs_binary.go

package main

import (
	"fmt"

	"github.com/gabriel-vasile/mimetype"
)

func main() {
	testBytes := []byte("This random text has a MIME type of text/plain; charset=utf-8.")
	detectedMIME := mimetype.Detect(testBytes)

	isBinary := true
	for mtype := detectedMIME; mtype != nil; mtype = mtype.Parent() {
		if mtype.Is("text/plain") {
			isBinary = false
		}
	}

	fmt.Println(isBinary, detectedMIME)
}