Fix converter 'image2pdf'

When an image is store in a document type that does not have the Picture facet (e.g. File), any feature that needs a conversion to PDF (e.g the PDF rendition, or the publication with PDF rendition) fails if the image is other than JPEG, PNG, or GIF. See converter image2pdf.

How to reproduce:

• create a document of type File with a TIFF image as main blob
• get its PDF rendition

Expected result: the download of the PDF rendition starts.
Actual result: the download of the PDF rendition does not start.

Workaround: deploy the following XML extention:

  <require>org.nuxeo.ecm.platform.convert.plugins</require>

  <extension target="org.nuxeo.ecm.core.convert.service.ConversionServiceImpl" point="converter">

    <converter class="org.nuxeo.ecm.platform.convert.plugins.Image2PDFConverter" name="image2pdf">
      <sourceMimeType>image/*</sourceMimeType>
    </converter>

  </extension>