Partially revert PR #2190

The core shouldn't depend on constants that are not defined by itself
diff --git a/system/libraries/Email.php b/system/libraries/Email.php
index fac8a49..1f2f6a9 100644
--- a/system/libraries/Email.php
+++ b/system/libraries/Email.php
@@ -732,7 +732,7 @@
 				return FALSE;
 			}
 
-			if ( ! $fp = @fopen($file, FOPEN_READ))
+			if ( ! $fp = @fopen($file, 'rb'))
 			{
 				$this->_set_error_message('lang:email_attachment_unreadable', $file);
 				return FALSE;