Try contacting them and explaining to them that you installed the work on the buyer’s server. Either through the help desk or email@example.com
If they agree the work was done they might remove the warning.
It doesn’t force someone to attach a file in the box and it doesn’t give an automatic account warning if you don’t attach something. In the past I’ve just added a dropbox link (when the files were very big) and text saying what was done, but it’s safest to attach something too (or just attach the work and the text saying what was done)…
You could have put a text message in the delivery box saying where the files were installed with maybe a link if that would be okay (if you wouldn’t get in trouble for the link).
In future if you can attach them as a .zip or something in the delivery box as well as installing them in the buyer’s site that would be safest I think. As well as putting some text in the delivery box saying what work has been done & where it’s installed.