CDN

• Path: server/{channel}/{attach}
• Preview path: server/{channel}/{attach}.preview.png

Videos:

1. During upload:
   • write video to a path;
   • use ffprobe to get video width and height;
   • use ffmpeg to extract preview frame and write it to preview path;
   • add record to DB (with non-zero width and height);
2. If client requests video w/o query args:
   • serve file as-is;
3. If client requests video w/ query args:
   • read file from preview path;
   • apply required changes;
   • serve that file;