I can fully understand why it's unsupported: It's unsafe for both Aternos and its users.
Aternos deliberately does not allow file uploads and requires a restricted list of mods, plugins, and modpacks to prevent anyone from uploading potentially malicious files to their system or something of that caliber. Aternos does not want that in any way, shape, or form. As a 100% free, ad-funded service, they host millions, probably even billions of Minecraft servers, and they don't want them all getting erased because a single server is successfully tricked into running a database-wiping file or two.
While I don't know how that'd even work, or even if it works that way at all, let's just leave it at that and move onto a more important point:
People value their privacy.