MM-36295: Do not convert thumbnails/previews to jpeg for png images (#21230)

We were applying a white background to transparent images
and converting them to jpegs. This was to make text be legible
behind a black preview background.

However, this has led to a poor user experience, as users rarely
download the full image but always click on previews. Therefore,
we need the previews to remain as pngs.

To fix this, we just re-encode them as pngs instead of jpgs.

```release-note
NONE
```

services/slackimport/slackimport.go

@@ -91,11 +91,11 @@ type Actions struct {
 	CreateGroupChannel     func(request.CTX, []string) (*model.Channel, *model.AppError)
 	CreateChannel          func(*model.Channel, bool) (*model.Channel, *model.AppError)
 	DoUploadFile           func(time.Time, string, string, string, string, []byte) (*model.FileInfo, *model.AppError)
-	GenerateThumbnailImage func(image.Image, string)
-	GeneratePreviewImage   func(image.Image, string)
+	GenerateThumbnailImage func(image.Image, string, string)
+	GeneratePreviewImage   func(image.Image, string, string)
 	InvalidateAllCaches    func()
 	MaxPostSize            func() int
-	PrepareImage           func(fileData []byte) (image.Image, func(), error)
+	PrepareImage           func(fileData []byte) (image.Image, string, func(), error)
 }
 
 // SlackImporter is a service that allows to import slack dumps into mattermost
@@ -793,13 +793,13 @@ func (si *SlackImporter) oldImportFile(timestamp time.Time, file io.Reader, team
 	}
 
 	if fileInfo.IsImage() && !fileInfo.IsSvg() {
-		img, release, err := si.actions.PrepareImage(data)
+		img, imgType, release, err := si.actions.PrepareImage(data)
 		if err != nil {
 			return nil, err
 		}
 		defer release()
-		si.actions.GenerateThumbnailImage(img, fileInfo.ThumbnailPath)
-		si.actions.GeneratePreviewImage(img, fileInfo.PreviewPath)
+		si.actions.GenerateThumbnailImage(img, imgType, fileInfo.ThumbnailPath)
+		si.actions.GeneratePreviewImage(img, imgType, fileInfo.PreviewPath)
 	}
 
 	return fileInfo, nil