fix user agent for desktop app (#6612)

app/login.go

@@ -6,6 +6,7 @@ package app
 import (
 	"fmt"
 	"net/http"
+	"strings"
 	"time"
 
 	"github.com/mattermost/platform/einterfaces"
@@ -90,6 +91,10 @@ func DoLogin(w http.ResponseWriter, r *http.Request, user *model.User, deviceId
 		bname = "unknown"
 	}
 
+	if strings.Contains(r.UserAgent(), "Mattermost") {
+		bname = "Desktop App"
+	}
+
 	if bversion == "" {
 		bversion = "0.0"
 	}