Files
Upload a file
Keeps a file you attach to many emails, like your terms or your logo: upload it once,
then pass its file_id in attachments. Send it as multipart/form-data in a file
field, or as JSON with filename and base64 content. Uploading the same content again
returns the existing file.
POST
Upload a file
Body
application/json
Response
200 - application/json
The response is of type string.

