bug#12972: [PATCH] Avoid regression in mailcap-view-file similar to Bug#44824

[Eli Zaretskii]

> From: Maxim Nikulin <manikulin@gmail.com>
> Date: Fri, 2 Jul 2021 23:24:23 +0700
> 
> On 02/07/2021 19:37, Eli Zaretskii wrote:
> >> From: Maxim Nikulin
> >> Date: Fri, 2 Jul 2021 19:21:55 +0700
> >> Cc: Lars Ingebrigtsen
> >>
> >>> And with other handlers, this could be an
> >>> incompatible behavior change if the handler behaves differently when
> >>> its standard handles are connected to a pipe rather than a terminal
> >>> device.
> >>
> >> Example of such handler, please.
> > 
> > Why do you need one?
> 
> Because of I can not imagine such case for mailcap handler in Emacs yet 
> and, accordingly to you, "this could be an incompatible behavior change".

You don't need to imagine it, you just need to trust me that I know
what I'm talking about: it would be an incompatible change.

Is it possible to detect that the handler is one of those that are
affected by this issue?  If so, let's do that; it cannot be worse than
what you suggested, or worse than the current situation.