Changed isUrlEncoded, to handle weird attachment names

Sometimes the filenames of attachments are completely converted, even alnum chars. 

This is a part from an example email, sent from thunderbird. The attachment name is Führerschein (driver's license):
...
Content-Disposition: attachment;
filename*0*=UTF-8''%46%C3%BC%68%72%65%72%73%63%68%65%69%6E;
...

This breaks the detection isUrlEncoded() and the resulting attachment filename is something similar to UTF-8''%46%C3%BC%68%72%65%72%73%63%68%65%69%6E

This commit uses another approach to detect if the filename is url encoded.