diff --git a/admin/tool/filetypes/edit_form.php b/admin/tool/filetypes/edit_form.php
index 5d16177212f..f2c60aa1260 100644
--- a/admin/tool/filetypes/edit_form.php
+++ b/admin/tool/filetypes/edit_form.php
@@ -94,6 +94,29 @@ class tool_filetypes_form extends moodleform {
         parent::set_data($data);
     }
 
+    public function get_data() {
+        $data = parent::get_data();
+
+        // Update the data to handle the descriptiontype dropdown. (The type
+        // is not explicitly stored, we just set or unset relevant fields.)
+        if ($data) {
+            switch ($data->descriptiontype) {
+                case 'lang' :
+                    unset($data->description);
+                    break;
+                case 'custom' :
+                    unset($data->corestring);
+                    break;
+                default:
+                    unset($data->description);
+                    unset($data->corestring);
+                    break;
+            }
+            unset($data->descriptiontype);
+        }
+        return $data;
+    }
+
     public function validation($data, $files) {
         $errors = parent::validation($data, $files);
 
@@ -110,6 +133,20 @@ class tool_filetypes_form extends moodleform {
             $errors['defaulticon'] = get_string('error_defaulticon', 'tool_filetypes', $extension);
         }
 
+        // If you choose 'lang' or 'custom' descriptiontype, you must fill something in the field.
+        switch ($data['descriptiontype']) {
+            case 'lang' :
+                if (!trim($data['corestring'])) {
+                    $errors['corestring'] = get_string('required');
+                }
+                break;
+            case 'custom' :
+                if (!trim($data['description'])) {
+                    $errors['description'] = get_string('required');
+                }
+                break;
+        }
+
         return $errors;
     }
 }
diff --git a/admin/tool/filetypes/tests/behat/add_filetypes.feature b/admin/tool/filetypes/tests/behat/add_filetypes.feature
index c36e434fd7c..ca52f01d2fb 100644
--- a/admin/tool/filetypes/tests/behat/add_filetypes.feature
+++ b/admin/tool/filetypes/tests/behat/add_filetypes.feature
@@ -36,6 +36,35 @@ Feature: Add customised file types
     And I press "Save changes"
     And I should see "frog" in the "application/x-7z-compressed" "table_row"
 
+  Scenario: Change the text option (was buggy)
+    Given I log in as "admin"
+    And I navigate to "File types" node in "Site administration > Server"
+    When I click on "Edit 7z" "link"
+    And I set the following fields to these values:
+      | Description type   | Custom description specified in this form |
+      | Custom description | New description for 7z                    |
+    And I press "Save changes"
+    Then I should see "New description" in the "application/x-7z-compressed" "table_row"
+    And I click on "Edit 7z" "link"
+    And I set the field "Description type" to "Default"
+    And I press "Save changes"
+    And I should not see "New description" in the "application/x-7z-compressed" "table_row"
+
+  Scenario: Try to select a text option without entering a value.
+    Given I log in as "admin"
+    And I navigate to "File types" node in "Site administration > Server"
+    When I click on "Edit dmg" "link"
+    And I set the field "Description type" to "Custom description"
+    And I press "Save changes"
+    Then I should see "Required"
+    And I set the field "Description type" to "Alternative language string"
+    And I press "Save changes"
+    And I should see "Required"
+    And I set the field "Description type" to "Default"
+    And I press "Save changes"
+    # Check we're back on the main page now.
+    And "dmg" "table_row" should exist
+
   Scenario: Delete an existing file type
     Given I log in as "admin"
     And I navigate to "File types" node in "Site administration > Server"