View xlsx file with Midnight Commander

mc.ext

# Add this to your mc.ext user file, search for
# Microsoft Excel Worksheet and _prepend_ this rule to the existing one.
regex/i/\.xlsx$
  Open=/usr/lib/mc/ext.d/doc.sh open msxls
  View=%view{ascii} /usr/local/bin/xlsx2csv %f
# Please note that if you do not prepend this block to the existing block for xls/xlsw, its regexp
# will catch the xlsx and so xlsx2csv will not be called
# Be sure to keep the existing viewer directives, as in this example:

# Microsoft Excel Worksheet
#regex/i/\.xlsx$
#  Open=/usr/lib/mc/ext.d/doc.sh open msxls
#  View=%view{ascii} /usr//bin/xlsx2csv %f
# Be sure to keep the existing viewer directives.

#regex/i/\.xl[sw]$
#        Open=/usr/lib/mc/ext.d/doc.sh open msxls
#        View=%view{ascii} /usr/lib/mc/ext.d/doc.sh view msxls
#type/^Microsoft\ Excel
#        Open=/usr/lib/mc/ext.d/doc.sh open msxls
#        View=%view{ascii} /usr/lib/mc/ext.d/doc.sh view msxls

If a xlsx2csv cannot be found anywhere in the system, you need to create a shell script with the correct path to your site-packages/.
For example on a Mac OS X system with Python 2.7 from MacPorts, after I installed xlsx2csv with pip, I had to create the /usr/local/bin/xlsx2csv script containing:

#!/bin/bash
python /opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/xlsx2csv.py $*