[nemo-qml-plugin-email] Simplify add attachments code.

Current implementation fails when the filename has no latin chars.
QMF implementation(addAttachmentsToMultipart) already handles this properly
encoding the name in utf-8 format.
It does not work, fails here, same as previous code: https://github.com/nemomobile-packages/messagingframework/blob/master/qmf/src/libraries/qmfclient/qmailmessage.cpp#L1528