opc: add content type for .docx package
opc: add content type for .docx package

Needed to set Content-Type header in an HTTP Response object carrying
a .docx payload.

mohamedattahri/python-docx,eruffaldi/python-docx,python-openxml/python-docx,jean/python-docx,holli-holzer/python-docx,SimplyAgree/python-docx,Nuos/python-docx,yurac/python-docx,tanyunshi/python-docx