[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.
Merge pull request #101 from nemomobile/attachImpro

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