Backend partials (widgets and controllers) are just PHP code.
Why not allow us to define _foo.php files instead of .htm?
Benefits?
PHP Syntax highlighting in editors.
ps: Sorry for the summarized text. No time to type detailed explanation here.
You mean that u need url ends with .php files instead of .htm ?
This has been discussed before. Technically it should be _foo.html.php and theme files should be page.html.php.twig.
We use the abandoned extension htm as an "October view file". The syntax highlighting should support: HTML, PHP and Twig combined. Sublime Text 3 had support for this but it recently broke in their latest build, still looking for a fix, we may need to create our own syntax highlighters. In the meantime we just use PHP + HTML syntax for htm files.
@mg-freelancer , @daftspunk already answered your question.
@mg-freelancer , just for you information, this is how PHPStorm currently shows partial backend files:

And this is how it should do:

Backend's Partial files are just plain PHP files. I'm pretty sure it's not too hard to locate both files.
+1. Both backend partials and frontend files use .htm extension. I basically have _twig_ enabled for .htm extensions in PhpStorm and my highlighting for backend files looks like on the first screenshot of @yourwebmaker
These files have different syntax, so it makes sense for them to use different extensions.
"There are plugins for sublime that will highlight everything" is not really the best argument, some of us still prefer IDE ;)
The hope is that one day someone will develop an October syntax parser for inline Twig + PHP together, bound to the .htm file extension. In which case, problem solved.