Add support for XPS conversion

[hancush]

XPS is a proprietary Microsoft document format similar to a PDF. They can't be opened in OpenOffice, so unoconv can't convert them. I spiked conversion with ConvertAPI in this branch. It was super easy, but it's not a free service. XPS attachments seem rare, so I'm going to gauge whether this is a must have or nice to have. (The old merger did not support this filetype, instead skipping attachments it couldn't convert.)

[hancush]

As a stopgap, I've updated the merger to skip attachments that are not of an expected type in 6e84626.