fix: improve multipart form handling in API client

- Updated the `prepareRequest` method to use `CreatePart` for file uploads, enhancing the handling of multipart form data.
- Set appropriate MIME headers for file uploads, including `Content-Disposition` and `Content-Type`, ensuring better compatibility with various file types.

These changes improve the robustness of file uploads in the API client, providing clearer specifications for the uploaded files.

Author: gibiw