Windows: Fails for files in root folder

[wilhelmberg]

Surfaced in:
mapbox/mapbox-studio-classic#1344

mapnik-omnivore cannot detect the file type, if the file is located in the root of the drive, e.g. directly in C:\.
I've tested and this is not an issue on Linux with files in /.

For csv it fails with Incompatible filetype.

For geojson it fails with Error obtaining layer names

This does not happen if the file is moved into a sub directory.