Not sure if your problem is already solved, but I came up with a new idea. First you need to https://www.mobileread.com/forums/showthread.php?t=81119
. Once installed start it and enter:
gconftool-2 -R /apps/er/sys/ctb/filetypes
On my machine the html-part was near the bottom, so I could see it, else do something like:
gconftool-2 -R /apps/er/sys/ctb/filetypes >/media/mmcblk0p1/log.txt
The keyboard and the small font of rxvt make it very hard to enter the line correctly, fortunately, the arrow keys work as cursor keys, so you can correct mistakes.
The emulator contains the following data regarding html:
descr = HTML
icon_name = html
generate_thumbnail = false
extensions = [mht,mhtml,htm,html,xhtml]
viewer_cmd = /usr/bin/erbrowser -n
Please check if you have something simular/equal.