skip non-file attachments

For other attachments such as links, there is no filename.
In the future, these could be handled more cleverly, e.g. by creating a file
containing a link.

indico-fetcher.py

@@ -111,6 +111,9 @@ def exec_request(indico_instance, request_path):
                 mod = attachment['modified_dt']
                 mtime = dateutil.parser.parse(mod)
                 mtimets = int(time.mktime(mtime.timetuple()))
+                if attachment['type'] != 'file':
+                    print('Skipping non-file attachment...')
+                    continue
                 full_filename = fulldir + "/" + str(mtimets) + "_" + attachment['filename']
 
                 if not os.path.isfile(full_filename):