uploadFile method
Upload a file to Anthropic
API Reference: https://docs.anthropic.com/en/api/files-create
Uploads a file to Anthropic's file storage. The file can then be referenced in messages for analysis or processing.
Implementation
@override
Future<FileObject> uploadFile(FileUploadRequest request) async {
  final formData = FormData();
  formData.files.add(
    MapEntry(
      'file',
      MultipartFile.fromBytes(
        request.file,
        filename: request.filename,
      ),
    ),
  );
  final responseData = await client.postForm('files', formData);
  return FileObject.fromAnthropic(responseData);
}