Mono repo -> Master (#22553)

Combines the following repositories into one:

https://github.com/mattermost/mattermost-server
https://github.com/mattermost/mattermost-webapp
https://github.com/mattermost/focalboard
https://github.com/mattermost/mattermost-plugin-playbooks

server/channels/app/imaging/preview.go

@@ -0,0 +1,66 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package imaging
+
+import (
+	"bytes"
+	"fmt"
+	"image"
+	"image/jpeg"
+
+	"github.com/disintegration/imaging"
+)
+
+// GeneratePreview generates the preview for the given image.
+func GeneratePreview(img image.Image, width int) image.Image {
+	preview := img
+	w := img.Bounds().Dx()
+
+	if w > width {
+		preview = imaging.Resize(img, width, 0, imaging.Lanczos)
+	}
+
+	return preview
+}
+
+// GenerateThumbnail generates the thumbnail for the given image.
+func GenerateThumbnail(img image.Image, width, height int) image.Image {
+	thumb := img
+	w := img.Bounds().Dx()
+	h := img.Bounds().Dy()
+	expectedRatio := float64(height) / float64(width)
+
+	if h > height || w > width {
+		ratio := float64(h) / float64(w)
+		if ratio < expectedRatio {
+			// we pre-calculate the thumbnail's width to make sure we are not upscaling.
+			targetWidth := int(float64(height) * float64(w) / float64(h))
+			if targetWidth <= w {
+				thumb = imaging.Resize(img, 0, height, imaging.Lanczos)
+			} else {
+				thumb = imaging.Resize(img, width, 0, imaging.Lanczos)
+			}
+		} else {
+			// we pre-calculate the thumbnail's height to make sure we are not upscaling.
+			targetHeight := int(float64(width) * float64(h) / float64(w))
+			if targetHeight <= h {
+				thumb = imaging.Resize(img, width, 0, imaging.Lanczos)
+			} else {
+				thumb = imaging.Resize(img, 0, height, imaging.Lanczos)
+			}
+		}
+	}
+
+	return thumb
+}
+
+// GenerateMiniPreviewImage generates the mini preview for the given image.
+func GenerateMiniPreviewImage(img image.Image, w, h, q int) ([]byte, error) {
+	var buf bytes.Buffer
+	preview := imaging.Resize(img, w, h, imaging.Lanczos)
+	if err := jpeg.Encode(&buf, preview, &jpeg.Options{Quality: q}); err != nil {
+		return nil, fmt.Errorf("failed to encode image to JPEG format: %w", err)
+	}
+	return buf.Bytes(), nil
+}