Lighttpd 1.4.40 made overriding keys in an existing array a hard error. The Stretch version of lighttpd is 1.4.45. This change in the upstream application makes the advice at https://wikitech.wikimedia.org/wiki/Help:Toolforge/Web/Lighttpd#Header,_mimetype,_character_encoding,_error_handler for replacing existing mime-type mappings with new local versions obsolete.
Prior to Debian I set the txt files on betacommand-dev to "".txt" => "text/plain;charset=UTF-8"," while the default config is '".txt" => "text/plain",'
This creates an issue with some UTF-8 files as web browsers render it as "Western" encoding
"Jean_Prouvé_(1981).jpg" is rendered as "Jean_ProuvÃ©_(1981).jpg" See this report: https://tools.wmflabs.org/betacommand-dev/reports/Multichill_files.txt
This is an issue with Debian because I can no longer override the default setting, instead I get an error message about having a duplicate key.