You are right Vincent, we are using a webapi to do that, but you can also do the same thing through base.

In your "old" instance, create a webapi with this method:

[RestExtension("Helper")]
public class Helper
{
    [RestExtensionMethod(returnXml=false)]
    public static string MediaPathById(string id) {
        return new Media(int.Parse(id)).getProperty("umbracoFile").Value.ToString();
    } 
}

In the "New" Instance, in your proxy method, replace each media fields in the legacy umbraco.config by the path to the file "c:/../../yourfile" (depend on your structure, you will have to download it first). Take an account that uMirror when the property is an mediaPicker expect a path to a file.

Let me know If you have any other question

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