Add UTI for `.gpx` files (so they are treated as `.xml`)

[mrdowdeswell]

I'd like to render .gpx files in the QL extension. Although public.xml appears in the mdls output below (so .gpx should be treated as .xml) it seems the extension stops at unknown com.topografix.gpx.

The UTI for .gpx files seems to be com.topografix.gpx. Not sure if this needs to be explicitly added?

Running mdls on a .gpx test file:

mdls -name kMDItemContentType -name kMDItemContentTypeTree test.gpx
kMDItemContentType     = "com.topografix.gpx"
kMDItemContentTypeTree = (
    "com.topografix.gpx",
    "public.xml",
    "public.text",
    "public.data",
    "public.item",
    "public.content"
)