Use remoteCommand for this. For e.g.:
<h:form id="idAssessmentsUploadForm" enctype="multipart/form-data">
<h:panelGrid cellspacing="10" styleClass="standard-panel" columns="2" id="idAssessmentsUploadPanelGrid">
<h:outputText value="#{msg['application.assessmentsUploadRequest.loader']}"/>
<p:selectOneMenu id="idLoader"
style="width: 230px;"
value="#{configurationBean.loaderName}"
required="true">
<f:selectItems value="#{configurationBean.loaders}"/>
</p:selectOneMenu>
</h:panelGrid>
<p:fileUpload fileUploadListener="#{configurationAction.processConfigurationUpload}"
allowTypes="/(\.|\/)(gif|jpe?g|png)$/"
required="true"
onstart="loadProperty()"
update="messages"
mode="advanced"/>
<p:remoteCommand name="loadProperty">
<f:setPropertyActionListener for="idLoader"
value="#{configurationBean.loaderName}"
target="#{configurationBean.loaderName}"/>
</p:remoteCommand>
</h:form>
Not tested but should work.