Decode filenames of messages forwarded as attachements. Propably similar to bug#2994865.
Can you please provide a test case and a more detailed explanation of how to reproduce? Feel free to attach a test message as well, if that helps.
Example messages + screenshots.
Affected are messages with diacritic in subject. Example: Subject: =?ISO-8859-2?B?rmx1u2916Gv9IGv58g==?=
Select checkbox beside this message in mailbox list and click on "Forward". Then filename of attached file is wrong.
Sorry, but I cannot reproduce the fix (I see anomalous encoding problems that I don't have time to look at myself right now, but your patch does not change that). Can you provide more information about your environment and what charset/language you are using when you test?
$squirrelmail_default_language = 'cs_CZ';
$default_charset = 'utf-8';
$lossy_encoding = false;
Probably variable default_charset is important. One more testing message is attached.