uri::split <path> "file" fails on Windows when the <path> contains a drive letter:

% uri::split file
scheme=C
unknown scheme 'C' in

In this case, the presence of the ':' is overriding the explicit choice of "file" for the scheme.
This use to work in version 1.2.1, but that appears to have been by accident because a regexp ignored uppercase letters.  A lowercase 'c' would fail the same way in the older release.

I would think the explicit "file" parameter should override any guessing by the code.