diff --git a/stringoverrides_migrate.admin.inc b/stringoverrides_migrate.admin.inc
index 8f1721c..7631bee 100644
--- a/stringoverrides_migrate.admin.inc
+++ b/stringoverrides_migrate.admin.inc
@@ -34,8 +34,11 @@ function stringoverrides_migrate_admin_import() {
  * Triggered when the user imports data.
  */
 function stringoverrides_migrate_admin_import_submit($form, &$form_state) {
-  // Check if the file uploaded correctly
-  $file = file_save_upload('file');
+  // Check if the file uploaded correctly.
+  $validators = array(
+    'file_validate_extensions' => array('po'),
+  );
+  $file = file_save_upload('file', $validators);
   if (!$file) {
     form_set_error('file', t('A file to import is required.'));
     return;