MM-38982: fixes SVG uploading (#20141)

* MM-38982: fixes SVG uploading

Uploading SVG files was "almost" treated like an image in some places,
resulting in not showing the SVG on the client.
This is happening because when we fail to generate a mini preview for an
image, we prepend "invalid-" to the MimeType.

This commit adds a check to guard against SVG files in methods that make
no sense for SVGs.

* Reverts invalid-{mimetype} fix

* Uses fileInfo.IsSvg() where it can

app/upload.go

@@ -263,7 +263,7 @@ func (a *App) UploadData(c *request.Context, us *model.UploadSession, rd io.Read
 	}
 
 	// image post-processing
-	if info.IsImage() {
+	if info.IsImage() && !info.IsSvg() {
 		if limitErr := checkImageResolutionLimit(info.Width, info.Height, *a.Config().FileSettings.MaxImageResolution); limitErr != nil {
 			return nil, model.NewAppError("uploadData", "app.upload.upload_data.large_image.app_error",
 				map[string]interface{}{"Filename": us.Filename, "Width": info.Width, "Height": info.Height}, "", http.StatusBadRequest)