Metadata extraction configuration

Hi,

I'm using the metadata extraction add-on, and it works, except that the 
plugin is used each time, whatever the mime type of the uploaded document.
I didn't change anything in the add-on, except extractions for my custom 
types.

Uploading a PDF File, the MSWordMDExt extraction, and the 
MSWordMDExtPlugin plug-in is used.

What is the use of the destinationMimeType attribute, and the 
sourceMimeType element ?

    <plugin name="MSWordMDExtPlugin"

class="org.nuxeo.ecm.platform.metadataext.plugins.MSWordMDExtractorPlugin"
      destinationMimeType="application/msword">
      <sourceMimeType>application/msword</sourceMimeType>
    </plugin>

Thanks again.

Xavier Pétard