Hi Carolina!

In case you are reading the file from your File System this could be an option:

engine.SendSLNetMessage(
new SendEmailMessage
{
To = [YOUR_E-MAIL_HERE] Body = "This an example e-mail! :)",
IsHtml = false,
Attachments = new[] {
new EmailAttachment
{
Data = File.ReadAllBytes([YOUR_FULL_FILE_PATH]),
FileName = [FILENAME_IN_THE_ATTACHMENT],
},
},
});

I've tested this with .csv and .txt files and seemed to work fine. Hope it helps!