Fix multi-byte filename

[gonwan]

Chm files with multi-byte file path are not shown under windows. Encode to utf-8.

[u-235]

Hi. If you have a sample file with this problem, please upload it for tests on other operating systems.

[gonwan]

test.zip
Find some time to create a minimum reproducible chm, since the original file is not permitted to share. It also works fine under Linux. For MacOS, I do not have a Mac at hand. But it can be treated as Linux I suppose.
without vs. with patch:

[u-235]

The case seems to be a bit more complicated than it may seem at first glance. As far as I know, the URL could not contain characters of national alphabets and as you can see on the screenshot, in Windows 7 the regular viewer does not show the problematic page either. And the proper way to solve such cases would be to make an additional setting with encoding choices. But as a quick solution your option is quite good.