diff --git a/system/libraries/Upload.php b/system/libraries/Upload.php
index 15caebe..f241837 100644
--- a/system/libraries/Upload.php
+++ b/system/libraries/Upload.php
@@ -526,6 +526,12 @@
 			$this->file_name = preg_replace('/\s+/', '_', $this->file_name);
 		}
 
+		if ($this->file_ext_tolower && ($ext_length = strlen($this->file_ext)))
+		{
+			// file_ext was previously lower-cased by a get_extension() call
+			$this->file_name = substr($this->file_name, 0, -$ext_length).$this->file_ext;
+		}
+
 		/*
 		 * Validate the file name
 		 * This function appends an number onto the end of
diff --git a/user_guide_src/source/changelog.rst b/user_guide_src/source/changelog.rst
index cde4b8b..1a06bd5 100644
--- a/user_guide_src/source/changelog.rst
+++ b/user_guide_src/source/changelog.rst
@@ -19,6 +19,7 @@
 -  Fixed a bug (#4407) - :doc:`Text Helper <helpers/text_helper>` function :php:func:`word_censor()` doesn't work under PHP 7 if there's no custom replacement provided.
 -  Fixed a bug (#4415) - :doc:`Form Validation Library <libraries/form_validation>` rule **valid_url** didn't accept URLs with IPv6 addresses enclosed in square brackets under PHP 5 (upstream bug).
 -  Fixed a bug (#4427) - :doc:`CAPTCHA Helper <helpers/capcha_helper>` triggers an error if the provided character pool is too small.
+-  Fixed a bug (#4430) - :doc:`File Uploading Library <libraries/file_uploading>` option **file_ext_tolower** didn't work.
 
 Version 3.0.4
 =============