[FIX] mail: mails with attachments containing special chars (such as é) were not delivered properly

get_filename method from email lib returns sometimes decoded value, sometimes still encoded.
for instance, get_filename cannot decode iso encoded filename

Recently, decode method has been applied on filename string, to handle iso encoded file. 
Sadly, one cannot decode already decoded values.
Therefore, decoding already decoded windows or utf-8 encoded filename with special chars failed.