multipart/form-data is one of the value of enctype attribute, which is used in form element that have a file upload. multi-part means form data divides into multiple parts and send to server.

What is multipart form data used for?

Multipart/form-data should be used for submitting forms that contain large files, non-ASCII data, and large binary data. Moreover, multipart/form-data can be used for forms that are presented using representations like spreadsheets, Portable Document Format, etc. i.e other than HTML.

What is a multipart document?

Multipart requests combine one or more sets of data into a single body, separated by boundaries. You typically use these requests for file uploads and for transferring data of several types in a single request (for example, a file along with a JSON object).