File objects have path generation issues (maybe Win32 specific)

[genehack]

In particular, it seems like $file->input_filename is ending up as an absolute path on Win32, which makes _build_url() generate nonsense paths (like C:\foo\bar\C:\foo\bar\baz.html) which leads to other things barfing down the road.

Probably worth documenting the relationship between input_filename, dest_dir, and url for different object types on *nix and Win32 and seeing what the pattern is.