Mark .3mf as supported by opendocument thumbnailer
.3mf (3D Manufacturing Format) is not exactly an "office document", but it often includes a thumbnail following the open packaging conventions.
I considered cloning the thumbnailer but that didn't seem like it'd make sense. There may also be some other formats with thumbnails that would be supported if added.
Sorry I just noticed there was also !216 but it seems stalled. I could switch to that approach if preferred.