Thanks again Wolfgang, with this setup everything is fine.

But I wanted to bring to the attention of Hans the inconsistency of the
prefix conversions by defaut, so that he can correct them:

I A chapter

[a figure] Figure I.1  % <-- Here the conversion of the prefix to
Romannumerals is done AUTOMATICALLY.

1.1 A section

[a figure] Figure 1.2 % <-- Here the conversion of the prefix to
Romannumerals is NOT done AUTOMATICALLY (the presence of a section before
seems to DISCARD THE AUTOMATIC CONVERSION). Nor in the prefix of the
section.


Code:

\setuphead[chapter][conversion=Romannumerals]
\starttext
\chapter{A chapter}
\placefigure{}{\externalfigure}
\section{A section}
\placefigure{}{\externalfigure}
\stoptext


Every prefix should be AUTOMATICALLY converted, not half of them. Hans,
could you please correct that in the next betas?

Best regards.