Fixes the handling of the filename parameter.

Uses the RE used by Ruby in cgi.rb, that handles
the embedded semicolons in the filename correctly.

Signed-off-by: Christian Neukirchen <chneukirchen@gmail.com>

lib/rack/utils.rb

@@ -449,7 +449,7 @@ def self.parse_multipart(env)
                 head = buf.slice!(0, i+2) # First \r\n
                 buf.slice!(0, 2)          # Second \r\n
 
-                filename = head[/Content-Disposition:.* filename="?([^\";]*)"?/ni, 1]
+                filename = head[/Content-Disposition:.* filename=(?:"((?:\\.|[^\"])*)"|([^;\s]*))/ni, 1]
                 content_type = head[/Content-Type: (.*)#{EOL}/ni, 1]
                 name = head[/Content-Disposition:.*\s+name="?([^\";]*)"?/ni, 1] || head[/Content-ID:\s*([^#{EOL}]*)/ni, 1]