Является ли file URL file:///c:/my абсолютным? Путь (path) абсолютен, но согласно определению этот URL является относительным:

2.3 Hierarchical schemes and relative links

In some cases, URLs are used to locate resources that contain
pointers to other resources. In some cases, those pointers are
represented as relative links where the expression of the location of
the second resource is in terms of "in the same place as this one
except with the following relative path". Relative links are not
described in this document. However, the use of relative links
depends on the original URL containing a hierarchical structure
against which the relative link is based.

Some URL schemes (such as the ftp, http, and file schemes) contain
names that can be considered hierarchical; the components of the
hierarchy are separated by "/".

здесь