HTML mail.

[peter@easthope.ca]

Hi,

Most messages with attachments are displayed in RoundCube Webmail with 
an openable menu presenting options Open and Download for each 
attachment.

Some HTML messages are opened or rendered directly.  No need to choose 
to Open the attachment.

At the syntax level, how do those two cases differ?  If I attach an 
HTML file to a message, what is needed for the recipient to see it 
rendered directly?  I'll guess that JavaScript is involved.

For me, nothing here hints at an explanation.
https://en.wikipedia.org/wiki/HTML_email

Thanks,                    ... P.

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