Replace urlencoded @ symbol

The fix for https://github.com/dutchcoders/transfer.sh/issues/215 led to @ being encoded as %40 in filenames in the URL returned, which is awkward when working with social media scrapes since ArchiveBot normalises it to @ again.

transfer.notkiska.pw-upload

@@ -1,2 +1,2 @@
 #!/bin/bash
-curl -s --upload-file "$1" https://transfer.notkiska.pw/; echo
+curl -s --upload-file "$1" https://transfer.notkiska.pw/ | sed 's,%40,@,g'; echo