Cloudfront resources are isolated to the FQDN
Opening https://dlnmh9ip6v2uc.cloudfront.net/datasheets/Dev/Arduino/Shields/MP3_Player_Example.pde results in SOCKS_USERNAME="dlnmh9ip6v2uc.cloudfront.net". But instead of isolating to the FQDN we should do so using the domain name, i.e. cloudfront.net.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information