When claws sends an attachment of unknown content-type, it uses: Content-Type: application/octet-stream; name=filename.kitten Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=filename.kitten Using the name= parameter is no longer necessary, so the following can be sent instead: Content-Type: application/octet-stream Content-Transfer-Encoding: base64 Content-Disposition: attachment; filename=filename.kitten Claws already behaves this way when it sends an attachment with a known content-type, so consistency should be preferred. Additionally, removing this name= parameter presents some message size savings.