Honestly, I don't remember what we ended up doing. We deal with so many web sites and that's been over a year ago. Sorry.
Thanks for the post. But I have sorted it out and successfully posted the HTML files. The reason it was not working is the files were copied to the folders at OS level instead of Sitefinity. When I uploaded the files in Sitefinity, it started to work.
However it's a little inconvenient to upload many files in Sitefinity because you have to select files one by one. It seems there's no way to select all and upload them together?
Thanks Sabrie for the reply and the video.
In my case I cannot upload files into library but into file system through administration->file structure->create folder...
From that interface, there's no option for selecting multiple files at one time.