[MM-28638] Improve image thumbnail generation logic (#15534)

* Improve image thumbnail generation logic

* Improve naming

app/image.go

@@ -0,0 +1,50 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+import (
+	"image"
+
+	"github.com/disintegration/imaging"
+)
+
+func genThumbnail(img image.Image) image.Image {
+	thumb := img
+	w := img.Bounds().Dx()
+	h := img.Bounds().Dy()
+
+	if h > ImageThumbnailHeight || w > ImageThumbnailWidth {
+		ratio := float64(h) / float64(w)
+		if ratio < ImageThumbnailRatio {
+			// we pre-calculate the thumbnail's width to make sure we are not upscaling.
+			targetWidth := int(float64(ImageThumbnailHeight) * float64(w) / float64(h))
+			if targetWidth <= w {
+				thumb = imaging.Resize(img, 0, ImageThumbnailHeight, imaging.Lanczos)
+			} else {
+				thumb = imaging.Resize(img, ImageThumbnailWidth, 0, imaging.Lanczos)
+			}
+		} else {
+			// we pre-calculate the thumbnail's height to make sure we are not upscaling.
+			targetHeight := int(float64(ImageThumbnailWidth) * float64(h) / float64(w))
+			if targetHeight <= h {
+				thumb = imaging.Resize(img, ImageThumbnailWidth, 0, imaging.Lanczos)
+			} else {
+				thumb = imaging.Resize(img, 0, ImageThumbnailHeight, imaging.Lanczos)
+			}
+		}
+	}
+
+	return thumb
+}
+
+func genPreview(img image.Image) image.Image {
+	preview := img
+	w := img.Bounds().Dx()
+
+	if w > ImagePreviewWidth {
+		preview = imaging.Resize(img, ImagePreviewWidth, 0, imaging.Lanczos)
+	}
+
+	return preview
+}