Modify URL in markdown link

I'm trying to modify a markdown file. In a file, there are many links like this one.

[string one](/stringtwo/#stringthree)

I'd like to change these to the following:

[string one](stringtwo.html#stringthree)

Remove slashes and add .html. I tried the following:

sed -i 's/](\(\/.*\)#/](\1.html#/g' file

But it returns [global configuration](/config/.html#globals). It doesn't remove slashes. How can I achieve this using bash or sed?