Files manager force file extension

[Yubin Ruan]

It seems that ubuntu(16.04LTS)'s file manager, whose name I don't know, will
stupidly force users to save file with a specific file extension. For example,
when I save the html page from `https://www.yahoo.com', I only got two options
in the file manager (at the lower right corner):
    "Webpage, Complete" and
    "Webpage, HTML only",
By default, I get `Yahoo.html' as the filename to save. I can change the
`Yahoo.html' to any name like `this-is-yahoo.html'. But I CANNOT save it to
something like `Yahoo.webpage' because the file manager will automatically
add a "html" file extension for me, thus making it `Yahoo.webpaeg.html'
eventually.

I think this behavior is ... not that rational :)

Anyone have any idea on how can I save something to arbitary name I want?
Or does anyone have any other file managers that do not force this behavior?

---
Yubin