How to write a custom API that accepts Multipart form data & converts that to base 64 in custom

Summary

How to write a custom API that accepts Multipart form data & converts that to base 64 in custom code?

Content

Hi,

I want to write a custom API that accepts multipart form data & some additional parameters as query parameter/ request body input. We have a connector API that accepts base64 for image upload.
So we want to accept multipart form data in our custom API, convert it to base64 internally in our code & call that connector API. Is there any example for custom code that accepts multipart form data in Custom APIs? How can we achieve this? we also want to accept some additional inputs in our API via uery/body parameters. Could someone please help us?