This is not the correct way to use IFileSystem's. I've fixed this all up in the rev: ded82611a7c328c64ed7707e3b5c9c8802dba7a4

GetRelativePath accepts a URL string (like /media/blah/blah.jpg) which converts it into the File System's relative path - which is generally the path that the FS will always use for everything. In this example that would be "blah\blah.jpg"

If you want to get the 'full' path to the file (and this is IFileSystem dependant) you'd get the FS's relative path and then use .GetFullPath