I've noticed this while digging into #26521 but afaict ParseMediaType behaves correctly according to RFC 2045 but the FormDataContentType doesn't return quoted-string. The function mentions that it formats according to Content-Type header but RFC 2616 Sect. 2.2 also excludes special characters
for use in token and must be part of a quoted-string as well.

What version of Go are you using (go version)?

go 1.10.3

Does this issue reproduce with the latest release?

Yes.

What did you do?

package main

import (
	"mime/multipart"
	"mime"
	"os"
	"fmt"
)

func main() {
	w := multipart.NewWriter(os.Stdout)
	err := w.SetBoundary("(boundary)")
	fmt.Println(err)
	ct := w.FormDataContentType()
	_, _, err = mime.ParseMediaType(ct)
	fmt.Println(ct, err)
}

(https://play.golang.org/p/5ZPC_EGHODn)

What did you expect to see?

multipart/form-data; boundary="(boundary)"

What did you see instead?

multipart/form-data; boundary=(boundary)

Which according to RFC 2045 and RFC 2616 is illegal as ( belongs to special characters that are only allowed in quoted-strings.