Attachments are temporary on the forum, so don't count on them being around forever.
Here are some places you can make sure your files stay around for years and years.FreeSpace FilesHunter
is graciously running this site for the community to upload not only their missions and campaigns, but their models, fan art, or anything else FreeSpace related.
You can log in with a public community account (username: freespace password: files), or you can register for your own repo where your files will be kept private.
Since it allows for direct linking, this can be used as a place to host files for the FSO Installer!MediaFire
You probably know these guys pretty well, they're a big online file host. Everyone uses them. There's a free sign up with good enough features. But the files download links will be indirect linking, which will not work with the FreeSpace Open Installer.
If someone has other file hosts that are reliable, post them here and I'll add them to the list.