I already have this kind of incident with Zenworks service desk 8.2 and it was the name of the file generated with French "accent" and filename with space. Almost from "Mac" user. maybe file with ' and ç in the name like "Document de François.docx" or "résultats de l'étude.xlsx. 

the file was uploaded to server and findable in linux shell but the web interface was unable to show or access it.

I remember that Microfocus made a fix for that in 8.3 version.

I finded my original submitted discussions

Attachment files with french é ç unable download file - Service Desk User Discussions - ZENworks Service Desk (microfocus.com)

@David, can you test with a file that does not have any special characters?

Your symptom is a little different, so I wonder if the "Fix" did not completely fix the issue?

And your version....Just make sure you actually ARE 8.3

Yes I am! Just triple-checked.

The file I was using was test.txt with the word "test" in it. I just tried now attaching a file to a ticket with a document called new.docx with some text in and still get the "file upload failed". I never used any strange characters and I'm using version 8.3.

Best I can do is suggest an SR or perhaps give 8.3.1 a shot.....