What is http multipart request?

As the official specification says, "one or more different sets of data are combined in a single body". So when photos and music are handled as multipart messages as mentioned in the question, probably there is some plain text metadata associated as well, thus making the request containing different types of data (binary, text), which implies the usage of multipart.

I have found an excellent and relatively short explanation here.

A multipart request is a REST request containing several packed REST requests inside its entity.

An HTTP multipart request is an HTTP request that HTTP clients construct to send files and data over to an HTTP Server. It is commonly used by browsers and HTTP clients to upload files to the server.

• What it looks like
• See Multipart Content-Type
• See multipart/form-data