Component ID changes after submit if encoding='multipart/form-data' and part of page has been included.

My test page:
<h:form enctype="multipart/form-data" id="_form">
<jsp:include page="menu.jsp" />
<t:inputFileUpload ></t:inputFileUpload>
<h:commandButton value="Upload"></h:commandButton>
</h:form>

menu.jsp :

<h:panelGrid columns="1" id="grid">
<h:selectOneMenu value="#

" id="select">
<f:selectItem itemValue="none" itemLabel="none"/>
<f:selectItem itemValue="www" itemLabel="www"/>
</h:selectOneMenu>
</h:panelGrid>

Firstly SelectOneMenu has '_form:select' id. After upload and page refreshed ID changed to '_form:selectj_id_1'.