SMTP send attachment - email

Question

I am learning networking recently.
It is known that SMTP only allows 7-bit ASCII.
I would like to know if SMTP can send attachment such as pictures, videos or files via email. If so, how can the attachment be sent?
Thank you

Answer 1

With SMTP, almost every email is using MIME (Multipurpose Internet Mail Extensions, RFC2045). MIME is independent of SMTP, but can be thought like extension to it. HTTP protocol is also using MIME.
MIME contains two parts. MIME headers and MIME content. MIME headers describe what the contents will be. MIME is full of features, but I'm only describing what is relevant to your question.
MIME headers of interest:
Content-Type describe what the content is. Simply, text is just text/plain and image/jpeg is JPEG image.
Content-Transfer-Encoding describe how the content is encoded. Common encodings in email are quoted-printable and base64. These encodings are designed for SMTP's 7bit delivery in mind.
Content-Disposition header contains for example attachments file name.
For sending attachment or multiple attachments, each attachment is encoded as described above (some headers + encoded content). First Content-Type header is set to multipart/mixed and given boundary tag like this:
Content-Type: multipart/mixed; boundary=4vvtTcl0dIpecpyi12ab54
And every attachment (headers + content) starts with boundary tag:
--4vvtTcl0dIpecpyi12ab54
followed by MIME headers and then contents. This way the email client can separate attachments from each other.
I would also like to mention that there is 8BITMIME extension to SMTP for 8 bit support. See RFC6152: SMTP Service Extension for 8-bit MIME Transport.