Fix nuxeo-document-preview behavior with ISO attachments

nuxeo-document-preview attempts to display ISOs. The conversion service spawns multiple soffice processes trying to convert to PDF. This should not happen.

To reproduce:

• download  http://archive.ubuntu.com/ubuntu/dists/disco/main/installer-amd64/current/images/netboot/mini.iso
• import in a new Workspace
• ps -ef ; observe soffice processes

It was observed that if the following is commented out of the nuxeo-document-preview element, the conversion service does not start soffice processes:

    <template mime-pattern=".*">
      <iframe id="frame" src="[[_iFrameSource]]"></iframe>
    </template>