TYPO3 Fluid Content Elements: Allow single content element only

<div xmlns="http://www.w3.org/1999/xhtml" lang="en"
     xmlns:f="http://typo3.org/ns/TYPO3/Fluid/ViewHelpers"
     xmlns:flux="http://typo3.org/ns/FluidTYPO3/Flux/ViewHelpers"
     xmlns:v="http://typo3.org/ns/FluidTYPO3/Vhs/ViewHelpers">

    <f:layout name="Container for Teammitglieder" />

    <f:section name="Configuration">
	<flux:form id="container-teammembers" options="{group: 'FCE Container'}">
            <flux:grid>
                <flux:grid.row>
                    <flux:grid.column name="left"
                                      variables="{allowedContentTypes: 'fluidcontent_content', Fluidcontent: {allowedContentTypes: 'Vendor.VendorFluidTest:Teammember.html'}}"
                                      />
                </flux:grid.row>
            </flux:grid>
	</flux:form>
    </f:section>

    <f:section name="Preview">
      <flux:widget.grid />
    </f:section>

    <f:section name="Main">
	<h3>Teammitglieder</h3>
        <flux:content.render area="left" />
    </f:section>
</div>