It's interesting that the example in the proposal uses the file: protocol rather than http. Was that a mistake? Today a browser would handle such a tag by looking for a local file, rather than "attempt[ing] to pull over the network."
I wonder how the browser distinguishes between a file residing on a host addressable at 10.1.1.10, and the local path /10.1.1.10/files/example.xlsx. Maybe it tries one and then the other if the first doesn't work.