gnus-mime-button-line-format

gnus-mime-button-line-format is a variable defined in `gnus-art.el'.
Its value is

"%{%([%p. %d%T]%)%}%e
"

Documentation:
Format of the MIME buttons.

Valid specifiers include:
%t The MIME type
%T MIME type, along with additional info
%n The `name' parameter
%d The description, if any
%l The length of the encoded part
%p The part identifier number
%e Dots if the part isn't displayed

General format specifiers can also be used. See Info node
`(gnus)Formatting Variables'.