/usr/share/cups/templates/option-pickmany.tmpl is in cups-server-common 1.7.5-11+deb8u2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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<TH {conflicted=1?CLASS="conflict":CLASS="label"} WIDTH="50%"><A NAME="{keyword}">{keytext}</A>:</TH>
<TD><SELECT NAME="{keyword}" MULTIPLE SIZE="10">
{[choices]<OPTION {choices={defchoice-1}?SELECTED:} VALUE="{choices}">{text}}
</SELECT></TD>
</TR>
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