How to retain a previously uploaded file?

I'm using a File input to allow uploading a file that is relevant to the record and all works as expected, except one critical thing:

When the item form is initially displayed for editing, a link to the file appears and works, but submitting other updates blanks out the field.
Surely this isn't expected behavior, so I'm not sure what I'm doing wrong. I want to be able to save the record, leaving the previously uploaded file intact.

Also, on a slightly related note, is there "somewhere", perhaps a plugin where the uploaded file can be processed at the time of upload (to extract certain metadata from within the file itself)?

FURTHER OBSERVATIONS:

I see that the originally uploaded file is being retained in a hidden input field, so I see that it "should" be retained, however something weird is happening:
<input type="hidden" name="jform[agenda_pdf][]" id="jform_agenda_pdf_hidden" value="0Agenda.pdf" />

When the Submit button is clicked, this hidden field is NOT included in the fields that are posted, thus the code in the table is blanking it out as it should. I see some javascript that is manipulating this value on submission, but I can't see to follow what is going on, so can't see why this field is being excluded from the POST.

Hello

I've the same problem. ¿Have you solved it?

Best regards

Santiago