There is no URL for a file. Basically the object you get is ItemFile with a following attributes (if this is a meta attribute and returned by ItemAttributes collection):
file.ContentType - describes content type of the file, like image/file and so on
file.FileContents - this is an actual binary content
file.FileName - name of the file.
These properties are set by ServicePipeHelper class and if you want, you can modify and add an URL. Otherwise, you you really want a URL, you will need to first save the FileContents property to a file and then redirect user to that URL.
Hope this helps.
PS: the issue mentioned above was resolved and is related to the Web Services in version 4.0.x.