gnus-article-browse-delete-temp

gnus-article-browse-delete-temp is a variable defined in `gnus-art.el'.
Its value is

ask

Documentation:
What to do with temporary files from `gnus-article-browse-html-parts'.
If nil, don't delete temporary files. If it is t, delete them on
exit from the summary buffer. If it is the symbol `file', query
on each file, if it is `ask' ask once when exiting from the
summary buffer.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.