primefaces p:fileUpload validation always shows default - validation

Question

I don't know how to custom validate primefaces p:fileUpload.
My code xhtml is:
<h:form id="agruparCVForm" styleClass="validacionform" enctype="multipart/form-data">
<p:messages id="messages" showDetail="false" autoUpdate="true" closable="true" escape="false"/>
<fieldset>
<div class="col">
<div class="dtPadding_bottom">
<p class="parrafoMargintop1em textSmall">
<h:outputText value="#{comunidadesBundle.textoAdjuntarDocumentoAsociar}" escape="false"/>
</p>
<p:fileUpload value="#{agruparComunidadController.comunidadAAgrupar.documentoAutorizacion}" mode="simple" skinSimple="true" required="true"/>
</div>
</div>
</fieldset>
<div class="float-cleaner marginTopBottom2em"></div>
<!-- Botones-->
<div class="ftr">
<h:commandButton value="#{comunidadesBundle.guardarCambiosBtn}" title="#{comunidadesBundle.guardarCambiosBtn}" action="#{agruparComunidadController.agrupar}" styleClass="buttonContrast azul textSmaller MarginBottom2em floatRMobile"/>
</div>
</h:form>
The action="#{agruparComunidadController.agrupar}" never gets called as the jsf validation is performed before getting the standard message from the api.
The question is how can I put a customized error message?
I'm new to primefaces and I don't know how it gets valiated.
I need to validate this field when the form get's submitted. So it should check if the field is empty and display the custom message instead of the standard message choose:.....
Thank you in advance.