From what I can tell, you're opening the file-stream, and then trying to open it again, before you close it.
Initial opening of the file:
FileStream _FileStream = new FileStream(apPath, FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite);
and
Response.TransmitFile(apPath);
seems to be trying to open the file again.
I would suggest calling
_FileStream.Close();
before calling TransmitFile
.