Currently, at least for the NSS v5.6.8, the If-None-Match header (see https://solidproject.org/TR/protocol#writing-resources) sis not supported. See nodeSolidServer/node-solid-server#1431. So in my upload method, I'm make sure to check to see if the file exists already if you want to make sure not to overwrite.
If this gets implemented, I should be able to take my overt lookup for the file out.