diff --git a/pkg/services/ngalert/notifier/channels/slack.go b/pkg/services/ngalert/notifier/channels/slack.go
index 57fee6e2541..71cc5440860 100644
--- a/pkg/services/ngalert/notifier/channels/slack.go
+++ b/pkg/services/ngalert/notifier/channels/slack.go
@@ -477,6 +477,11 @@ func (sn *SlackNotifier) createImageMultipart(image ngmodels.Image, channel, com
 	if err != nil {
 		return nil, nil, err
 	}
+	defer func() {
+		if err := f.Close(); err != nil {
+			sn.log.Error("Failed to close image file reader", "error", err)
+		}
+	}()
 
 	fw, err := w.CreateFormFile("file", image.Path)
 	if err != nil {
diff --git a/pkg/services/ngalert/notifier/channels/slack_test.go b/pkg/services/ngalert/notifier/channels/slack_test.go
index 98fa6278b8a..8b15177da54 100644
--- a/pkg/services/ngalert/notifier/channels/slack_test.go
+++ b/pkg/services/ngalert/notifier/channels/slack_test.go
@@ -409,6 +409,7 @@ func setupSlackForTests(t *testing.T, settings string) (*SlackNotifier, *slackRe
 	f, err := os.Create(t.TempDir() + "test.png")
 	require.NoError(t, err)
 	t.Cleanup(func() {
+		_ = f.Close()
 		if err := os.Remove(f.Name()); err != nil {
 			t.Logf("failed to delete test file: %s", err)
 		}