Hello!
I am using the php SDK to send an MMS with an image attachment (I tried using jpeg/png images). The documentation at https://developers.ringcentral.com/guide/messaging/sms/sending-images#php says: “Therefore, you must use multipart form-data, and set the content-type header to multipart/mixed.” In the same page of the documentation there is an example in php that uses the MultiPartBuilder class from the SDK. However, in the SDK MultiPartBuilder class the content-type header is set to multipart/form-data, not mixed:

The documentation makes it sound as if I can only use multipart/mixed. So which content-type headers can I use to send an MMS?