Hi, Yerim

We do have an AddAttachment API for composing messages. It's documented here (you'll need to substitute your mail server address):

https://mail.your-mail-domain.com/Documentation/api#/reference/SmarterMail.Web.Api.MailController/AddAttachment/post

This call is generally used for inline attachments, hence the need for a ContentID. If you leave that part off, it MAY add a file as a standard attachment, but we've not tested that. (We're in the process of it.) One thing to note is that any file that's uploaded needs to be done using form data.