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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Bcc: peter at e a s t h o p e dot c a
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--- PyGate Linux v1.5.2
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

On Thu, Jan 22, 2026 at 08:17:56AM -0700, peter@easthope.ca wrote:

Hi,

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

Attachments are just one special case of MIME multipart messages.
Another would be "alternatives" -- just (in some way) the same
stuff presented in different ways, say as plain text and as HTML
(to stay with your example). That could also be PDF, Libre Office
Writer... you name it.
In the alternatives case, your mail user agent chooses whatever
is most "convenient" (something you can often configure).
See the MIME description here [1] for the whole kaboodle.
My guess is that those offered for download are "attachments" (i.e.
have the "Content-Disposition: attachment" in MIME parlance, whereas
those being shown have "Content-Disposition: inline" and are (one
of) several alternatives.
Does your mail client have a way to show you the mail "as is"?
Cheers
--
tom s


--- PyGate Linux v1.5.2
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

From: <tomas@tuxteam.de>
Date: Thu, 22 Jan 2026 18:17:54 +0100

My guess is that those offered for download ... have the "Content-Disposition: attachment" ... those being shown have "Content-Disposition: inline" ...

Does your mail client have a way to show you the mail "as is"?

Yes, can easily see plain text message "sources". Will check the Content-Disposition parameters.

Thanks! ... P.L.


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mobile: +1 778 951 5147
VoIP: +1 778 508 0020
Bcc: peter at e a s t h o p e dot c a
projects: en.wikibooks.org/wiki/User:PeterEasthope

--- PyGate Linux v1.5.2
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

On Thu, 22 Jan 2026 18:17:54 +0100
<tomas@tuxteam.de> wrote:

On Thu, Jan 22, 2026 at 08:17:56AM -0700, peter@easthope.ca wrote:

Hi,

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

Attachments are just one special case of MIME multipart messages.
Another would be "alternatives" -- just (in some way) the same
stuff presented in different ways, say as plain text and as HTML
(to stay with your example). That could also be PDF, Libre Office
Writer... you name it.

In the alternatives case, your mail user agent chooses whatever
is most "convenient" (something you can often configure).

See the MIME description here [1] for the whole kaboodle.

My guess is that those offered for download are "attachments" (i.e.
have the "Content-Disposition: attachment" in MIME parlance, whereas
those being shown have "Content-Disposition: inline" and are (one
of) several alternatives.

Does your mail client have a way to show you the mail "as is"?

I don't see anything earlier in this thread, so if it's RoundCube, then
yes, it can show the source. I use it for the odd occasion when I want
to see HTML from a trusted sender, as I normally run a client with text
only and no HTML plugin installed.

--
Joe

--- PyGate Linux v1.5.2
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

From: <tomas@tuxteam.de>
Date: Thu, 22 Jan 2026 18:17:54 +0100

My guess is that those offered for download are "attachments" (i.e.
have the "Content-Disposition: attachment" in MIME parlance, whereas
those being shown have "Content-Disposition: inline" and are (one
of) several alternatives.

Makes perfect sense.

A message marketing "Detox patches" is rendered graphically. These
are all of the Content_ header lines.

Content-Type: multipart/related;
type="multipart/alternative";

Content-Type: multipart/alternative;

Content-Type: text/plain;
charset="windows-1251"
Content-Transfer-Encoding: quoted-printable

Content-Type: text/html;
charset="windows-1251"
Content-Transfer-Encoding: quoted-printable

Content-Type: image/jpeg;
name="ijyhrlieicuow.jpeg"
Content-Transfer-Encoding: base64
Content-ID: <5cf8d01dc8c358a0b62840bed8b7cd@oltyssk>

No Disposition or disposition. Something more subtle is involved.

Thanks, ... P.

--
mobile: +1 778 951 5147
VoIP: +1 778 508 0020
Bcc: peter at e a s t h o p e dot c a
projects: en.wikibooks.org/wiki/User:PeterEasthope

--- PyGate Linux v1.5.5
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

On Sat 24 Jan 2026 at 14:14:40 (-0700), peter@easthope.ca wrote:

From: <tomas@tuxteam.de>
Date: Thu, 22 Jan 2026 18:17:54 +0100

My guess is that those offered for download are "attachments" (i.e.
have the "Content-Disposition: attachment" in MIME parlance, whereas
those being shown have "Content-Disposition: inline" and are (one
of) several alternatives.

Makes perfect sense.

A message marketing "Detox patches" is rendered graphically. These
are all of the Content_ header lines.

[ ? ]

No Disposition or disposition. Something more subtle is involved.

Perhaps the ones that display graphically are being told to do so
by some HTML code in an attachment, rather than by the image's own
Content-foo headers (assuming that's possible).

Cheers,
David.

--- PyGate Linux v1.5.6
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)