I assume you are using the File plugin to write to a file in your app. Unfortunately, as the plugin ReadMe
states, the FileWriter.write(blob)
method is not supported in Windows Phone. The authors may extend it in the future to support this feature but we cannot tell if and when.
One option to resolve the problem is to host the files somewhere else where they are downloadable as files. If they aren't too large, you can consider storing them in Telerik Backend Services
Alternatively, if you have knowledge of C#, you can research what is needed in order to extend the File plugin
to support writing a file on Windows Phone. After you modify the source code provided in the plugin repository, you can import it as a custom plugin
in AppBuilder and use it in your app.