What protocol is used to send email attachments?

Question

Communication protocols

I can't find the answer to the question. Interested in all possible options - sending a letter through a mail client (the letter goes via SMTP), sending a letter through a browser (HTTP\HTTPS), other options (using SPDY), etc.

Answer 1

As far as I know, SMTP is without options (an exception is attachments in Gdrive/mailru/etc, when the file is placed in the cloud, and only a link to it is attached to the letter).