SendMail, GMail Send-Feature and others should support a Content-ID for attachments. This can be optionally set for each attachment.
I want to send an E-Mail with an attachment (image). This is image should be embedded within an IMG-Tag and shown in the E-Mail.
Attached Images can be rendered within HTML-E-Mails. Inline Base64 does not work with GMail.
See CID Using attachments – Nodemailer
Sorry, not a programmer anymore.