diff --git a/server/upstream/helper.go b/server/upstream/helper.go
index 8847317..5bbe833 100644
--- a/server/upstream/helper.go
+++ b/server/upstream/helper.go
@@ -53,7 +53,7 @@ func (o *Options) getMimeTypeByExtension() string {
 	}
 	mimeType := mime.TypeByExtension(path.Ext(o.TargetPath))
 	mimeTypeSplit := strings.SplitN(mimeType, ";", 2)
-	if forbidden := o.ForbiddenMimeTypes[mimeTypeSplit[0]]; forbidden || mimeType == "" {
+	if o.ForbiddenMimeTypes[mimeTypeSplit[0]] || mimeType == "" {
 		if o.DefaultMimeType != "" {
 			mimeType = o.DefaultMimeType
 		} else {