attachments for transfers (type 4) are not base58 encoded

[sbrekelmans]

According to the docs, the attachment should be base58 encoded, but it seems they are not.

https://docs.ltonetwork.com/protocol/public/transactions/transfer-transaction

You can reproduce this problem by using the ledger to send a transaction and include a plain-text attachment. You can view the attachment (using a swagger api and the tx id). It should be base58 encoded.