In order to send certain characters in an email they need to be encoded. With the attachment name this is what you're seeing. We are encoding it into Base64 when sending it out. However, the Base64 text you provided is invalid as it contains an underscore at the end. As well, fixing the encoded text and decoding it reads [χειακά και στο.docx] instead of the original file name you specified.
Is the encoded text you sent completely unmodified, and does it come from the renaming of the file you specified or another?
Also, are you seeing the encoded text in SmarterMail or another client?