Add comprehensive unit tests for FileApiClient covering:
- Route verification tests for all 4 download endpoints (POST method)
- Route verification tests for 2 upload endpoints (POST method)
- Success tests for downloads returning byte arrays
- Success tests verifying existing data can be passed to downloads
- Success tests for uploads returning IReadOnlyList<T>
- Multipart content-type and filename verification test
- Error tests (404 for download, 400 validation error for upload)
Joseph Doherty