fileUploadField value not showing up in the Process when using a!startProcess

I have a simple test Process Model with two parameters, recordId (Text) and uploadedDoc (Document). I want to call this Process from a non-submit button in my interface so in the saveinto, I have this:

saveInto: {
  a!startProcess(
    processModel: cons!DAA_UPLOAD_FILE_PROCESS_MODEL,
    processParameters: {
      recordId: ri!recordId,
      uploadedDoc: ri!uploadedDoc
    }
  )
}

The ri!uploadedDoc is populated by a fileUploadField component in the interface:

a!fileUploadField(
  label: "File Upload",
  labelPosition: "ABOVE",
  target: cons!DAA_UNFILED_ATTACHMENT_FOLDER,
  maxSelections: 1,
  value: ri!uploadedDoc,
  saveInto: ri!uploadedDoc
)

So when the button is pressed, the Process starts as normal but only the recordId process variable is passed to the Process. The uploadedDoc parameter is always empty. Does this mean that the fileUploadField component does not work with a!startProcess in the Interface? Or is there a way to achieve this?

There's no issue when using the same fileUploadField component and then I just use a Submit button... the Process accepts the uploaded document this way. But in my particular user case, it would be nicer if the user can issue multiple fileUploads without submitting the form yet.