How to create Signature V4 authentication header when uploading files as multipart/form-data in REST API of API Gateway?

Dears,

I am trying to authenticate REST API with IAM in API Gateway. I created Signature V4 directly in Java Client, and both GET and POST requests were successful. However, in the case of API request (POST) for file upload, multipart/form-data should be used, but I cannot find the exact information on how to create Signature V4 for file upload through API Gateway. How to create Signature V4 authentication header when uploading files as multipart/form-data in REST API of API Gateway?