I think the only REAL way to have longevity of uploaded files would be for StackExchange to host a file server for this purpose. The problem with Host Your Own is that the user might take it down and the SE links will 404. This includes Dropbox, Amazon S3, Skydrive, and any other service that is maintained by a user subscription.

until he closes his S3 account. There needs to be something a little more permanent.