Note valid backup file extensions in error toast

app/src/main/res/values/strings.xml

@@ -369,7 +369,7 @@
     <string name="tracker_not_logged_in">Not logged in: %1$s</string>
     <string name="backup_created">Backup created</string>
     <string name="invalid_backup_file">Invalid backup file</string>
-    <string name="invalid_backup_file_type">Invalid backup file: %1$s</string>
+    <string name="invalid_backup_file_type">Invalid backup file type: %1$s\nIt should end with ".proto.gz" or ".json".</string>
     <string name="invalid_backup_file_missing_data">File is missing data.</string>
     <string name="invalid_backup_file_missing_manga">Backup does not contain any manga.</string>
     <string name="backup_restore_missing_sources">Missing sources:</string>