Cannot upload txt file nor PDF when it contains some unknown character

This is a new fault. I cannot upload a TXT file nor a PDF file. I have tried adding a File Attachment to an invoice as I do every month, but that fails. I also tried Receipt Hub Document Manager but that fails too; it shows me an HTTP 406 error code.

Please fix this or tell me how to work around it. I attach receipts to Invoices so my client can see them.

Grateful for your help. Toodle pip, Duncan

I did a bit more testing and found that my document is the problem: it contains something that the upload function doesn’t accept. I haven’t found what it is.