Added staging file previews support

3rd_party_apps/filetransfers/backends/base_url.py

@@ -0,0 +1,7 @@
+from django.conf import settings
+
+def public_download_url(file, **kwargs):
+    """
+    Directs downloads to a handler at settings.PUBLIC_DOWNLOADS_URL_BASE
+    """
+    return settings.PUBLIC_DOWNLOADS_URL_BASE + file.name