How can I properly use multipart/form-data format in RESTful API Client

We are trying to do API end to end testing and part of our testing is attachment/upload API but we’re block in testing it and always getting an error. Below is the error we always encounter

Server responded with status code 400 instead of expected 200 status code.
Server returned response body:
{“errors”:{"":[“Failed to read the request form. Missing content-type boundary.”]},“type”:“RFC 7231 - Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content or more validation errors occurred.”,“status”:400,“traceId”:“00-177865efaf7b1b4ea9c925f2ac57060b-16e7e84947adbc4d-00”}

Would you please help our team to solve this and tell us the proper way of using multipart/form-data. Thanks