MDL-68435 customfield: fix plugin file serving callbacks.

Incorrect parameter/method call was previously serving files with
wrong filenames.

customfield/field/textarea/lib.php

@@ -72,5 +72,5 @@ function customfield_textarea_pluginfile($course, $cm, $context, $filearea, $arg
     }
 
     // We can now send the file back to the browser - in this case with a cache lifetime of 1 day and no filtering.
-    send_file($file, 86400, 0, $forcedownload, $options);
+    send_stored_file($file, DAYSECS, 0, $forcedownload, $options);
 }

customfield/lib.php

@@ -82,6 +82,5 @@ function core_customfield_pluginfile($course, $cm, $context, $filearea, $args, $
     }
 
     // We can now send the file back to the browser - in this case with a cache lifetime of 1 day and no filtering.
-    // From Moodle 2.3, use send_stored_file instead.
-    send_file($file, 86400, 0, $forcedownload, $options);
+    send_stored_file($file, DAYSECS, 0, $forcedownload, $options);
 }