WhatsApp PDF caption vs filename

[Morgan Collett]

When I send a PDF as an attachment over WhatsApp, text is required in the body of the message, which seems to be for the filename:

However, the PDF displays on both iPhone and Android with a "." as the caption:

The channel log shows this as the message sent:

{"to":"_______","type":"document","document":{"id":"____","caption":"Brochure1.pdf","filename":"."}}

I've tried this with both an uploaded PDF and a link to a PDF.

Other platforms seem to be able to get this right - how do I add the message content to the filename?

Thanks

Morgan