http3: sniff HTTP Content Type (#3715)

* add sniff

* add test for sniff

* fix typo in comment

* move bodyAllowedForStatus() to top of the function to aviod calling it twice

* add comments

* format with gofumpt

* fix typo and simplify the code

http3/response_writer.go

@@ -80,10 +80,26 @@ func (w *responseWriter) WriteHeader(status int) {
 }
 
 func (w *responseWriter) Write(p []byte) (int, error) {
+	bodyAllowed := bodyAllowedForStatus(w.status)
 	if !w.headerWritten {
+		// If body is not allowed, we don't need to (and we can't) sniff the content type.
+		if bodyAllowed {
+			// If no content type, apply sniffing algorithm to body.
+			// We can't use `w.header.Get` here since if the Content-Type was set to nil, we shoundn't do sniffing.
+			_, haveType := w.header["Content-Type"]
+
+			// If the Transfer-Encoding or Content-Encoding was set and is non-blank,
+			// we shouldn't sniff the body.
+			hasTE := w.header.Get("Transfer-Encoding") != ""
+			hasCE := w.header.Get("Content-Encoding") != ""
+			if !hasCE && !haveType && !hasTE && len(p) > 0 {
+				w.header.Set("Content-Type", http.DetectContentType(p))
+			}
+		}
 		w.WriteHeader(http.StatusOK)
+		bodyAllowed = true
 	}
-	if !bodyAllowedForStatus(w.status) {
+	if !bodyAllowed {
 		return 0, http.ErrBodyNotAllowed
 	}
 	df := &dataFrame{Length: uint64(len(p))}