MM-14030 Add format and frame count to image metadata (#10757)

* MM-14030 Add initial CountFrames

* MM-14030 Add format and frame count to image metadata

* Fix tests and stop using image dimensions from OpenGraph

* Fix copyright header

* Move license to NOTICE.txt

utils/imgutils/gif_test.go

@@ -0,0 +1,80 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package imgutils
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/mattermost/mattermost-server/utils/testutils"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestCountFrames(t *testing.T) {
+	header := []byte{
+		'G', 'I', 'F', '8', '9', 'a', // header
+		1, 0, 1, 0, // width and height of 1 by 1
+		128, 0, 0, // other header information
+		0, 0, 0, 1, 1, 1, // color table
+	}
+	frame := []byte{
+		0x2c,                   // block introducer
+		0, 0, 0, 0, 1, 0, 1, 0, // position and dimensions of the frame
+		0,                      // other frame information
+		0x2, 0x2, 0x4c, 0x1, 0, // encoded pixel data
+	}
+	trailer := []byte{0x3b}
+
+	t.Run("should count the frames of a static gif", func(t *testing.T) {
+		var b []byte
+		b = append(b, header...)
+		b = append(b, frame...)
+		b = append(b, trailer...)
+
+		count, err := CountFrames(bytes.NewReader(b))
+
+		assert.Nil(t, err)
+		assert.Equal(t, 1, count)
+	})
+
+	t.Run("should count the frames of an animated gif", func(t *testing.T) {
+		var b []byte
+		b = append(b, header...)
+		for i := 0; i < 100; i++ {
+			b = append(b, frame...)
+		}
+		b = append(b, trailer...)
+
+		count, err := CountFrames(bytes.NewReader(b))
+
+		assert.Nil(t, err)
+		assert.Equal(t, 100, count)
+	})
+
+	t.Run("should count the frames of an actual animated gif", func(t *testing.T) {
+		b, err := testutils.ReadTestFile("testgif.gif")
+		require.Nil(t, err)
+
+		count, err := CountFrames(bytes.NewReader(b))
+
+		assert.Nil(t, err)
+		assert.Equal(t, 4, count)
+	})
+
+	t.Run("should return an error for a non-gif image", func(t *testing.T) {
+		b, err := testutils.ReadTestFile("test.png")
+		require.Nil(t, err)
+
+		_, err = CountFrames(bytes.NewReader(b))
+
+		assert.NotNil(t, err)
+	})
+
+	t.Run("should return an error for garbage data", func(t *testing.T) {
+		_, err := CountFrames(bytes.NewReader([]byte("garbage data")))
+
+		assert.NotNil(t, err)
+	})
+}