my mac mail has started encoding wrong mime type for word docs and mac notes

Recently my mac mail has started giving some outgoing file attachments (particularly Word and Excel files) the default mime type 'application/octet-stream' instead of the correct mime type e.g. 'application/msword'. It does this with 'Note' documents too.

This doesnt happen if the attachment is sent within the word processing program itself but only when sent as an attachment to Mac Mail.

Any suggestions please!