Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 11025)
+++ wp-includes/functions.php	(working copy)
@@ -2011,12 +2011,14 @@
  * @return string New filename, if given wasn't unique.
  */
 function wp_unique_filename( $dir, $filename, $unique_filename_callback = null ) {
-	$filename = strtolower( $filename );
+	// sanitize the file name before we begin processing
+	$filename = sanitize_file_name($filename);
+
 	// separate the filename into a name and extension
 	$info = pathinfo($filename);
 	$ext = !empty($info['extension']) ? $info['extension'] : '';
 	$name = basename($filename, ".{$ext}");
-
+	
 	// edge case: if file is named '.ext', treat as an empty name
 	if( $name === ".$ext" )
 		$name = '';
@@ -2028,12 +2030,8 @@
 		$number = '';
 
 		if ( !empty( $ext ) )
-			$ext = strtolower( ".$ext" );
+			$ext = ".$ext";
 
-		$filename = str_replace( $ext, '', $filename );
-		// Strip % so the server doesn't try to decode entities.
-		$filename = str_replace('%', '', sanitize_title_with_dashes( $filename ) ) . $ext;
-
 		while ( file_exists( $dir . "/$filename" ) ) {
 			if ( '' == "$number$ext" )
 				$filename = $filename . ++$number . $ext;
Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 11025)
+++ wp-includes/formatting.php	(working copy)
@@ -564,27 +564,27 @@
 }
 
 /**
- * Filters certain characters from the file name.
+ * Sanitizes a filename replacing whitespace with dashes
  *
- * Turns all strings to lowercase removing most characters except alphanumeric
- * with spaces, dashes and periods. All spaces and underscores are converted to
- * dashes. Multiple dashes are converted to a single dash. Finally, if the file
- * name ends with a dash, it is removed.
+ * Removes special characters that are illegal in filenames on certain 
+ * operating systems and special characters requiring special escaping 
+ * to manipulate at the command line. Replaces spaces and consecutive 
+ * dashes with a single dash. Trim period, dash and underscore from beginning
+ * and end of filename.
  *
  * @since 2.1.0
  *
- * @param string $name The file name
- * @return string Sanitized file name
+ * @param string $filename The filename to be sanitized
+ * @return string The sanitized filename
  */
-function sanitize_file_name( $name ) { // Like sanitize_title, but with periods
-	$name = strtolower( $name );
-	$name = preg_replace('/&.+?;/', '', $name); // kill entities
-	$name = str_replace( '_', '-', $name );
-	$name = preg_replace('/[^a-z0-9\s-.]/', '', $name);
-	$name = preg_replace('/\s+/', '-', $name);
-	$name = preg_replace('|-+|', '-', $name);
-	$name = trim($name, '-');
-	return $name;
+function sanitize_file_name( $filename ) {
+	$filename_raw = $filename;
+	$special_chars = array("?", "[", "]", "/", "\\", "=", "<", ">", ":", ";", ",", "'", "\"", "&", "$", "#", "*", "(", ")", "|", "~", "`", "!", "{", "}");
+	$special_chars = apply_filters('sanitize_file_name_chars', $special_chars, $filename_raw);
+	$filename = str_replace($special_chars, '', $filename);
+	$filename = preg_replace('(\s+|-+)', '-', $filename);
+	$filename = trim($filename, '.-_');
+	return apply_filters('sanitize_file_name', $filename, $filename_raw);
 }
 
 /**