Conversion to text for fulltext search
=======================================

text/plain
text/csv
application/csv
  cat '%s'

application/pdf
  pdftotext -nopgbrk %s - | sed -e 's/ [a-zA-Z0-9.]\{1\} / /g' -e 's/[0-9.]//g'

application/vnd.openxmlformats-officedocument.wordprocessingml.document
  docx2txt '%s' -

application/msword
  catdoc %s

application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
  xlsx2csv %s

application/vnd.ms-excel
  xls2csv %s

text/html
  html2text %s

Many office formats
  unoconv -d document -f txt --stdout '%s'

Conversion to pdf for pdf preview
==================================

text/plain
text/csv
application/csv
application/vnd.oasis.opendocument.text
application/msword
application/vnd.wordperfect
text/rtf
  unoconv -d document -f pdf --stdout -v '%f' > '%o'

image/png
image/jpg
image/jpeg
  convert -f pdf -density 300 '%f' '%o'

application/vnd.ms-powerpoint
application/vnd.openxmlformats-officedocument.presentationml.presentation
application/vnd.oasis.opendocument.presentation
  unoconv -d presentation -f pdf --stdout -v '%f' > '%o'

application/vnd.ms-excel
application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
application/vnd.oasis.opendocument.spreadsheet
  unoconv -d spreadsheet -f pdf --stdout -v '%f' > '%o'

Conversion to png for preview images
=====================================

If you have problems running convert on PDF documents then read this page
https://askubuntu.com/questions/1081895/trouble-with-batch-conversion-of-png-to-pdf-using-convert
It basically instructs you to comment out the line

<policy domain="coder" rights="none" pattern="PDF" />

in /etc/ImageMagick-6/policy.xml

image/jpg
image/jpeg
image/png
  convert -resize %wx '%f' '%o'

application/pdf
  gs -dBATCH -dNOPAUSE -sDEVICE=png16m -dPDFFitPage -r72x72 -sOutputFile=- -dFirstPage=1 -dLastPage=1 -q '%f' | convert -resize %wx png:- '%o'

text/plain
  a2ps -1 -a1 -R -B -o - '%f' | gs -dBATCH -dNOPAUSE -sDEVICE=png16m -dFirstPage=1 -dLastPage=1 -dPDFFitPage -r72x72 -sOutputFile=- -q - | convert -resize %wx png:- '%o'

application/msword
application/vnd.oasis.opendocument.spreadsheet
application/vnd.oasis.opendocument.text
application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
application/vnd.ms-excel
application/vnd.openxmlformats-officedocument.wordprocessingml.document
text/rtf
application/vnd.ms-powerpoint
text/csv
application/csv
application/vnd.wordperfect
  unoconv -d document -e PageRange=1 -f pdf --stdout -v '%f' | gs -dBATCH -dNOPAUSE -sDEVICE=pngalpha -dPDFFitPage -r72x72 -sOutputFile=- -dFirstPage=1 -dLastPage=1 -q - | convert -resize %wx png:- '%o'