Only one file transfer operation is supported per connected Host session.

The function initiates the file transfer and immediately returns control to your Windows HLLAPI application. This frees your application to perform other tasks while the file transfer is occurring. Once initiated the function will regularly post WinHLLAPIAsyncFileTransfer messages to the window specified by hWnd. These messages will notify the WinHLLAPI application of the status of the transfer and send a final message when the transfer is complete.

Is the status indicator: the high byte contains the Session ID, the low byte contains the status. If the low byte is zero, the file transfer is still in progress. If the low byte is one, the file transfer has completed.
If the low byte of wParm is zero (in progress), lParm is the number of bytes transferred. If the low byte wParm is one (completed), lParm is the completion code.