To make sure that your plugin overrides the original extension you have to provide the

Here is a working example of such an override.

Save it in a file

A note on the Content-Type

It is also worth to note that your xml files have to be valid xml with a correct xml version declaration ( ) so that they can be recognized as being of the correct MIME type: text/xml. Otherwise they could be recognized as text/plain, in spite of the file name extension, and then again processed as notes in spite of your valid override definition.

What happens when a file is uploaded (creating a Note, a Picture, a File or a custom type) is configured through the FileManager component, based on the mime type of the uploaded file. See http://explorer.nuxeo.org/nuxeo/site/distribution/current/viewContribution/org.nuxeo.ecm.platform.filemanager.service.FileManagerService.Plugins--plugins

in 5.4.2 (I believe this has been changed in 5.5) the default behavior for xml mile-type file is indeed to create Note, because of the following native contribution:

<extension point="plugins" target="org.nuxeo.ecm.platform.filemanager.service.FileManagerService">
<plugin class="org.nuxeo.ecm.platform.filemanager.service.extension.NoteImporter" name="NoteImporter"    order="10">
  <filter>text/plain</filter>
  <filter>text/html</filter>
  <filter>application/xhtml+xml</filter>
  <filter>application/xml</filter>
  <filter>text/xml</filter>
  </plugin></extension>

You can override this contribution with the same name (NoteImporter) and withdraw text/xml.