I don't know why it wasn't there already, it's in the mhshow
man page.
mhshow-suffix-image/png: .png
mhshow-suffix-image/tiff: .tif
mhshow-suffix-image/tiff: .tiff
mhshow-suffix-image/png: .png
mhshow-suffix-image/tiff: .tif
mhshow-suffix-image/tiff: .tiff
+mhshow-suffix-text: .txt
mhshow-suffix-text/calendar: .ics
mhshow-suffix-text/css: .css
mhshow-suffix-text/html: .html
mhshow-suffix-text/calendar: .ics
mhshow-suffix-text/css: .css
mhshow-suffix-text/html: .html