FIX receive_fe in case of base64 files coming from SDI. Without this …

…change, every SDI file would be doubly encoded in base64.

file_name_content_dict must be base64-encoded, as described in its documentation.
Next methods must handle the "real" content, in order to process ZIP files also.

l10n_it_sdi_channel/models/sdi.py

@@ -165,7 +165,7 @@ def receive_fe(
         for file_name, file_content in file_name_content_dict.items():
             attachments_values = self._process_single_fe(
                 file_name,
-                file_content,
+                base64.decodebytes(file_content),
                 **default_values,
             )
             if attachments_values: