Add charset to content type in getHandler (#545)

Add charset to content type in the getHandler function to fix CJK-letter related issues.
If the content type is empty after trimming, set it to "text/plain; charset=utf-8".

server/handlers.go

@@ -1218,7 +1218,7 @@ func (s *Server) getHandler(w http.ResponseWriter, r *http.Request) {
 			So add text/plain in this case to fix XSS related issues/
 		*/
 		if strings.TrimSpace(contentType) == "" {
-			contentType = "text/plain"
+			contentType = "text/plain; charset=utf-8"
 		}
 	} else {
 		disposition = "attachment"