Is it a Virus? : " HTMLViewer " For one time in my live, I accidentally set the option "Always" (tick the box) when Android ask me the question "which application must be used  to open this type of file?" 
Android suggested some text application and " HTMLViewer".



I found out this was not a good idea at all!

I tried to change the default application but " HTMLViewer" does not appear in the applications list, even if system files are displayed.

" HTMLViewer" is not known on Google as it seems, and I am now suspecting it perhaps is some kind of malware. 

I only installed apps from "Play Store".. 

Does anyone know something about this, and maybe solution to stop this behaviour? 



Here are my device's specs. As you can see... We can not update more ☺️





The inoperant changes done with "Better Open with" :
HTML Viewer is not a virus and you shouldn't be alarmed.
Actually it is a core application in most android versions (It's also present on my device running android 4)
HTML viewer (as the name suggests) basically lets you read HTML files stored on your device and it occasionally handles some .txt or related extensions of files present on your device and normally appears alongside other .txt handling applications.
Under normal circumstances you should be able to remove defaults of the HTML Viewer as an individual app:

Go to Settings → Apps → Tap 3 dot menu → Show system.
Select HTML Viewer → Open by default → Clear defaults:

  
You also can reset default applications by following this procedure:

Go to Settings → Apps → Tap 3 dot menu → Reset app preferences then click Reset Apps to confirm:


Alternatively as beeshyams suggested: using an app like Better Open With provides flexibility to choose the prefered app even if its one.
Update
Lib Caris is right by claiming that &quot;even by clearing defaults/resetting preferences&quot; HTML Viewer still opens .txt despite these interventions, and I agree with him.
Don't know if I can call it a bug, I reproduced this issue again and even if defaults are cleared the changes are not confirmed in android (Android 6).
Nevertheless I have two workarounds to bypass this &quot;bug&quot;:
1. Temporarily disabling HTML Viewer

Go to Settings → Apps → Tap 3 dot menu → Show system.
Select HTML Viewer → Disable:



After disabling HTML Viewer, navigate to your device storage and look for any .txt file and open it.
You will notice it will open with any available text viewer (not HTML Viewer)
Next follow the above procedure and re enable HTML Viewer.
Then try opening the .txt file again, this time around the defaults are cleared automatically and you have a chance to choose your preferred text viewer:


2. Choose open as with file manager

Alternatively, directly navigate to any .txt file on your device storage with a suitable file manager e.g ES explorer .
Long press the .txt file and choose &quot;Open As&quot;  → Clear defaults


Final Update
Due to problems in language barrier, it was a challenge to provide a conscise solution, nevertheless I will express my sincere gratitude to Lib Car, in his quest to address the problem and identifying pointer that ultimately solved this issue:

I did not find it since the beginning because it's name is NOT &quot;
HTMLViewer &quot;.  The NAME is &quot; Visionneuse HTML &quot; :