Looking at your Postman screenshot you can try setting the FieldMetadata collection to blank and try setting the FileMetadata collection's Content-Type as image/jpeg or application/octet-stream
Have you tried setting the Content-Transfer-Encoding: base64 after the Content-Type . You can look up few other threads for the format https://community.blueprism.com/t5/Product-Forum/Embed-a-image-in-email-body/m-p/94211#M43620
