How to Display Non-Folderish Documents (Files, Note, ...) in the Left Tree

The left tree is built using a query that is ran recursively. One of the clauses of the query makes sure that only folderish documents are displayed (AND ecm:mixinType = 'Folderish'). The query is defined in the "tree_children" page provider. See below a sample override of this query without the "Folderish" clause. You can contribute to the "providers" extension using the XML Extensions feature.

<require>org.nuxeo.ecm.webapp.pageproviders.contrib</require>

  <extension target="org.nuxeo.ecm.platform.query.api.PageProviderService"
    point="providers">

    <coreQueryPageProvider name="tree_children">
      <property name="maxResults">PAGE_SIZE</property>
      <pattern>
        SELECT * FROM Document WHERE ecm:parentId = ? AND ecm:isProxy = 0 AND
        ecm:mixinType != 'HiddenInNavigation'
        AND ecm:isVersion = 0 AND ecm:isTrashed = 0
      </pattern>
      <sort column="dc:title" ascending="true" />
      <pageSize>50</pageSize>
    </coreQueryPageProvider>
</extension>

You can adapt this sample so as to filter anything you would like to see or not in the left tree.