As mentioned in the XML snippet you quote, your service receives a URL, which has the form (actual ids in the URL are longer, and I shortened them below to make this more readable):
/fr/service/persistence/crud/UFWoD/Test1/data/e619/21c6.bin
Your service can retrieve the file by making a call to the Orbeon Forms persistence API, which means making an HTTP GET to:
http://server.com/orbeon/fr/service/persistence/crud/UFWoD/Test1/data/e619/21c6.bin
In the above URL, replace http://server.com/orbeon
by the actual URL on which your Orbeon Forms is deployed. Also, note that since version 4.0, access to services is protected by default. For more on this, see Authorization of pages and services.