/usr/share/mh/xlbiff.form is in xlbiff 4.1-7.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | %<(mymbox{to})*%| %>\
%(void(rclock{date}))\
%<(gt 15768000)%03(month{date})%02(year{date})%|\
%<(gt 604800)%02(mday{date})%03(month{date})%|\
%<(gt 86400) %(day{date}) %|\
%02(hour{date}):%02(min{date})%>%>%>\
%<{date} %|*%>\
%<(mymbox{from})To:%14(friendly{to})%|%17(friendly{from})%> \
%{subject}%<{body}<<%{body}%>
|