EmailMessage.addAttachment

The attachmentFileName is what is shown to the user, not the file on your sending computer. It should NOT have a path in it. If you want a filename from your computer, try addFileAsAttachment.

The mimeType can be excluded if the filename has a common extension supported by the library.

message.addAttachment("text/plain", "something.txt", std.file.read("/path/to/local/something.txt"));