I used to work at a place like that, it was incredibly frustrating and time-consuming. So I came up with a solution that works even if S3 is blocked: I built https://github.com/OkGoDoIt/UploadAndPaste and set up SCP file hosting on my own server that listens on port 443. (They blocked most outgoing connections to non-standard ports and did MITM sniffing on any port 80 traffic, so this was the only way to get through.) Then I could just easily "paste" a file to a my remote server and download on the other machine via a url.