Thank you for the suggestion Hraban.

The source comes from Markdown, which is converted to XML, then typeset
using ConTeXt. There's no Markdown-specific mechanism to relate images to a
particular external figure definition, unfortunately. All images use the
same syntax and are treated the same way. The only part that differs is the
file name extension (and header within the file).

One possibility would be something like:

\setupexternalfigure[
    width=1em,
    height=1em,
    order={svg,pdf,png,jpg},
    location={local,default,global},
]
\defineexternalfigure[png][
    width=\textwidth,
    height=\textheight,
]
\unprotect
\let\old_externalfigure=\externalfigure
\tolerant\def\externalfigure[#1]#,[#2]#,[#3]{%
    \doifelseinstring{.png}{#1}{%
        \old_externalfigure[#1][png][#2]%
    }{%
        \doifelsefileexists{#1.png}{%
            \old_externalfigure[#1.png][png][#2]%
        }{%
            \old_externalfigure[#1][#2][#3]%
        }%
    }%
}\protect

See: https://tex.stackexchange.com/a/650221/2148

I was wondering if there was a simpler or a solution that's more the
ConTeXt-way?

Thanks again!