Return null if file already exists locally and system
 does not want to receive again. Throwing IOException will
 result in the transfer aborting.  Both of these results in a nack
 being sent to the sender.</div>
<dl>
<dt><span class="paramLabel">Parameters:</span></dt>
<dd><code>fileName</code> - remote sender's preferred name for the file</dd>
<dd><code>transferName</code> - remote sender's logical name for the transfer</dd>
<dd><code>sha256hash</code> - sha256 hash of the file, as reported by the sender<