Can't save a pdf to a files field

I have a block with a files field:

icon: download
    label: File
    type: files
    max: 1
    uploads: pdf

and a file template in the Files blueprint folder called pdf.yml

title: Pdf
  extension: pdf, doc, docx

I upload the pdf, it looks like it has saved, but when I refresh the page it has gone. Any ideas why? Images save just fine.