FEATURE: Show video thumbnail in composer (#27233)

When uploading a video, the composer will now show a thumbnail image in
the composer preview instead of just the video placeholder image.

If `enable_diffhtml_preview` is enabled the video will be rendered in
the composer preview and is playable.

lib/pretty_text/helpers.rb

@@ -44,6 +44,10 @@ module PrettyText
 
       urls.each do |url|
         sha1 = Upload.sha1_from_short_url(url)
+        if (url.split(".")[1].nil?) # video sha1 without extension for thumbnail
+          thumbnail = Upload.where("original_filename LIKE ?", "#{sha1}.%").last
+          sha1 = thumbnail.sha1 if thumbnail
+        end
         map[url] = sha1 if sha1
       end