This part can definitely be confusing — when you choose the Azure DevOps connector, there’s no option for "Transfer files into destination application," which is required for handling attachments. We need to add those file upload endpoints to our metadata model (I created an internal ticket for that).
In the meantime, there’s a workaround:
Choose the HTTP connector
Select "Transfer files into destination application"
Set the request media type to Octet stream. In the Advanced section, set the path to your file’s blob key. If you're pulling the file from a lookup or somewhere other than the default file input, make sure to reference the correct key name (e.g., blobKey, or whatever field holds the file).
I haven’t tested this myself yet, but based on the Azure DevOps docs you linked, this should work with raw binary and doesn’t require Base64 encoding.