Docx reader: support extracting embedded non-image files with --extract-media

[orthoxerox]

I often receive docx documentation with embedded files. It would be useful if Pandoc could extract them and replace them with hyperlinks, similar to what it does with images.

The files are stored in word\embeddings and their representations in document.xml look like this:

<!-- Object embedded as icon -->
<w:object w:dxaOrig="1543" w:dyaOrig="998">
  <v:shapetype id="_x0000_t75" coordsize="21600,21600" o:spt="75" o:preferrelative="t" path="m@4@5l@4@11@9@11@9@5xe" filled="f" stroked="f">
    <v:stroke joinstyle="miter"/>
    <v:formulas>
      <v:f eqn="if lineDrawn pixelLineWidth 0"/>
      <v:f eqn="sum @0 1 0"/>
      <v:f eqn="sum 0 0 @1"/>
      <v:f eqn="prod @2 1 2"/>
      <v:f eqn="prod @3 21600 pixelWidth"/>
      <v:f eqn="prod @3 21600 pixelHeight"/>
      <v:f eqn="sum @0 0 1"/>
      <v:f eqn="prod @6 1 2"/>
      <v:f eqn="prod @7 21600 pixelWidth"/>
      <v:f eqn="sum @8 21600 0"/>
      <v:f eqn="prod @7 21600 pixelHeight"/>
      <v:f eqn="sum @10 21600 0"/>
    </v:formulas>
    <v:path o:extrusionok="f" gradientshapeok="t" o:connecttype="rect"/>
    <o:lock v:ext="edit" aspectratio="t"/>
  </v:shapetype>
  <v:shape id="_x0000_i1025" type="#_x0000_t75" style="width:77.25pt;height:49.9pt" o:ole="">
    <v:imagedata r:id="rId6" o:title=""/>
  </v:shape>
  <o:OLEObject Type="Embed" ProgID="Word.Document.12" ShapeID="_x0000_i1025" DrawAspect="Icon" ObjectID="_1675674465" r:id="rId7">
    <o:FieldCodes>\s</o:FieldCodes>
  </o:OLEObject>
</w:object>

<!-- Inline embedding -->
<w:object w:dxaOrig="9355" w:dyaOrig="450">
 <v:shape id="_x0000_i1028" type="#_x0000_t75" style="width:467.65pt;height:22.5pt" o:ole="">
   <v:imagedata r:id="rId8" o:title=""/>
 </v:shape>
 <o:OLEObject Type="Embed" ProgID="Word.Document.12" ShapeID="_x0000_i1028" DrawAspect="Content" ObjectID="_1675674466" r:id="rId9">
   <o:FieldCodes>\s</o:FieldCodes>
 </o:OLEObject>
</w:object>

The r:id attribute can be used to locate the correct embedded file via _rels\document.xml.rels:

<Relationship Id="rId7" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/package" Target="embeddings/Microsoft_Word_Document.docx"/>
<Relationship Id="rId9" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/package" Target="embeddings/Microsoft_Word_Document1.docx"/>