Sent messages with attachment names consisting of special characters, such as ÅÄÖ, are improperly displayed when opening the message in the Sent folder. Instead of anmälan av löntagaruppgifter.pdf the name appears to be iso-8859-1''anm%E4lan%20av%20l%F6ntagaruppgifter%2Epdf If I click on the attachment and open it in Okular, the wrongly displayed name appears in the Save File dialog window if I try to save a copy of the attachment. I am guessing that the name is probably correctly encoded to with the percentage signs and everything, but that it is not properly decoded when the message is presented in the Sent folder.
I tested here and it's ok. I can't reproduce your bug
The example I provided was from an e-mail message sent on June 2, 2017. I just tried again, and I couldn't reproduce the problem either. Perhaps it has already been fixed? Nevertheless, I am sorry for wasting your time.