Getting the right syntax is vital for the attachment JSON. For anyone else, this is what finally worked for me:
return [{
"data": get_FileProperties.data.base64Data,
"name": get_FileProperties.data.name,
"contentType": 'text/csv'
}]
Since learning this I have now constructed a workflow to loop through all flight data records and email them to all our pilots using the retool workflow component to include a report as well as a csv attachment of their flights for the month.