The attachment data. If the base64 field is true, this data must be encoded using base64. Otherwise, it will be interpreted as UTF-8.
base64
boolean
If set to true, data needs to be encoded using base64. Otherwise data will be interpreted as UTF-8.
content_type
string
content_id
string
If specified, this attachment will be added as an inline attachment and a multipart/related MIME container will be generated in the message to hold it and the textual content.