How to embed attachment in email body

Use the cid: URI scheme

--boundary-example 1
Content-Type: Text/HTML; charset=US-ASCII

to the other body part, for example through a statement such as:
<IMG SRC="cid:foo4*foo1@bar.net" ALT="IETF logo">

--boundary-example-1

Content-ID: <foo4*foo1@bar.net>
Content-Type: IMAGE/GIF
Content-Transfer-Encoding: BASE64

R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
etc...

--boundary-example-1--

References

• Can an HTML email body reference a file sent as an attachment (in the same email)?
• Content-ID and Message-ID Uniform Resource Locators
• Embedding Images in HTML Email: Have the Rules Changed?