diff --git a/contented.go b/contented.go
index 3708e66..e27ca3a 100644
--- a/contented.go
+++ b/contented.go
@@ -121,7 +121,7 @@ func (this *NTFServer) ContentedUploadSync(fileId string, expectSizeBytes int64)
 	if err != nil {
 		return "", err
 	}
-	req.Header.Set("Content-Type", `multipart/form-data`)
+	req.Header.Set("Content-Type", formValues.FormDataContentType()) // it's "multipart/form-data; boundary=xxxx" with a magic string
 	uploadResp, err := http.DefaultClient.Do(req)
 	if err != nil {
 		return "", fmt.Errorf("Upload failed: %s", err.Error())