Datagrid datanavigateurlformatstring

Question

How can you set the datanavigateurlformatstring dynamically so each row in a datagrid contains a unique URL?  I have a database with a filename field:
x.doc
y.doc
z.doc
Each of the filenames in the filename field refer to a file saved in a directory on the webserver:
/savedfiles/x.doc
/savedfiles/y.doc
/savedfiles/z.doc

I want to display in each row of the datagrid a unique url referencing the files so the user can download each file:

Can I create datanavigateurlformatstring dynamically to do this?