I changed the configuration setting on "file Extensions for viewing. " for viewing html/htm file on browser. however, i wouldn't make it work.
Is this setting is working to view html/htm files? (please see the attachment)
There are two settings which allows file viewables:
1. Set "Show View Button" config option to Yes.
2. Add file extensions of that file type to the File Extensions for viewing config option.
After that, when you view a document from front-end, you should see a View button which allows you to click on to view the document without downloading it.