As the description in bug 1545080 comment #6 says - Close TB, start cmd.exe as admin and run this command:
reg delete "HKLM\SOFTWARE\Clients\Mail\Mozilla Thunderbird" /v "SupportUTF8" /f
reg delete "HKLM\SOFTWARE\Wow6432Node\Clients\Mail\Mozilla Thunderbird" /v "SupportUTF8" /f
Try to send mails with attached files contains polish characters in their names - "ąśćźżęóńł ĄŻŚŹĆĘÓŁŃ".
works on TB 60.5.1, 60.5.2, 60.5.3, 60.6.0 and 60.6.1.
But because installer of TB for now cannot remove this value from registry automaticly - see bug 1545362.
You must remove this manually after TB is updated.
60.5.2 sets SupportUTFP8 to 1,
60.5.3 - 60.6.1 sets SupportUFTP8 to 0
on some machines that is enough that sets it to 0, but on my ~200 machines that i manage, i must to remove this value from registry all the time, when TB is updated.