Not quite, because it still runs the “hello world” app instead of trying to start an app contained in .html file. And it’s even weirder with ID file, which instantly starts Hello World App, even when new instance of test.gridnet is open and I do not login as myself.
One more thing - if I open “Hello world” app, and then try to open .txt file it runs Hello World. When I do rightclick-open with-editor, it opens in editor. After closing and leftclicking on the same .txt file it opens Hello World again.
trying to reproduce these things right now…
From what I understand, the described behaviour can be experienced only after the Hello World UI dApp was run at least once, is that correct?
If so,
The “Hello World” UI dApp registered an implicit file handler, for any kind of file containing ‘bytes’, despite of the actual file extension.
I assume it is also ‘by design’ since the ID file is a file containing a byte-array as well, and thus would trigger the file handler registered by the “Hello World” UI dApp.
Let me try to explain. I went through the described situation all over again and recorded a video clip 
Yea, I understand that. But when I use “open with…” and open .html file with an editor, doesn’t it assign said editor as a “default” handler now?
Or when I run “Hello world” once, it overwrites file handlers forever?
So, @Alpacalypse, my friend (!), I went ahead and tried to reproduce the situation.
As the app gets launched the file handler gets registered.
From now on, it makes the file you’ve mentioned (“ID” file) being opened through the “Hello World” UI dApp.
Then on the very same video, we see a counter example.
An attempt is made to open the ‘GNC’ file, which under the hood is being stored as a file of type ‘unsigned big integer’, it is a ‘byte vector’ no more 
The file handler registered by ‘Hello World’ UI dApp is thus not being chosen by the ‘App Selector’ system component.
Another one is chosen instead,- one owned by the ‘Editor’ UI dApp.
The result is, the GNC file is opened in Editor, just as one would expect.
Then, by the end of the video, we may see how one can override the default file handler through the context sub-menu.
That is not the case. As of now, it is a one-time thing which only overrides the current default handler, yet again - one time only.
We probably could consider having such a functionality implemented i.e. some sort of a checkbox instructing the system to treat the selected app as default, for the type of a selected file.
Under the hood, the system would then go ahead and register an explicit file handler on its own.
Yes, that is how it is as of now.
We probably should modify the “Hello World” UI dApp to prevent confusion 
after it gets launched.
@TheOldWizard kindly take a look.
While I do no think, it is a priority for now, the functionality certainly needs to be implemented. Feel free to open a ticket. Also, altering the handlers registered by the ‘Hello World’ app to be less ‘invasive’ might be a good idea indeed, so to eliminate confusion.