On https://dev.freeagent.com/docs/bank_transaction_explanations, might be worth explicitly stating that it’s necessary to strip any base 64 preamble (eg data:image/png;base64,) for the data field for attachments:
-
data(binary data of the file being attached encoded as base64)
If you forget to do this and you get the error “attachment is not a supported format”. (Leaving this here for anyone else who has made that mistake 
).