Editing the "Open with" menu in Windows 10

For anyone that still has this problem, the solution would be going to

\HKEY_CLASSES_ROOT\Applications\"ProgramName.exe"\SupportedTypes

and add a new string containing the format you want to open e.g. ".mp4"

I had the exact same problem when I couldn't add Nitro Pro to the "Open with" Context Menu for *.pdf files. That's on Win7, but it doesn't look like if MS' approach to the feature has changed between both Operating Systems.

Trying out all kind of suggestions found online, I also gave NirSoft's OpenWithView a shot. (see http://www.nirsoft.net/utils/open_with_view.html ) This tool lists "all available applications in the 'Open With' dialog-box of Windows". Sure enough, it also listed Nitro Pro, but I became suspicious because the default icon of the tool wasn't displayed in the overview screen, whereas for all other programs it was. Next doublechecking the application's path (that's another column in the same overview), I found that it listed an old path, not the current one. (I had changed a folder name in the path at some point, an was surprized to find it back in this overview)

With yet another NirSoft tool, MUI Cacheview (see http://www.nirsoft.net/utils/muicache_view.html ), which allows to "view and edit the list of all MuiCache items on a system", I found that the MUIcache contained several values for Nitro Pro. Not only the most recent (and correct) one: somehow the obsolete name had also pertained. Apparently, this erroneous/obsolete value was the first one encountered and withheld by other applications, which could explain the abnormal behavior of the program.

As soon as OpenWithView displayed Nitro Pro correctly (i.e. correct path, also reflected by the tool's icon being shown), I was able to add it to the "Open With" list by following the steps indicated already: right click a pdf file >> Open with >> Choose another app (or "Choose another program..." in Win7). Next browse to the correct location, select the executable, and click "Open" button.

I'm not claiming that the same is happening in OP's case, as several elements of the "Open with" menu seem to behave bizarrely (see e.g. also this other post Strange open with list (inconsistent with registry) mentioned by OP), but it's not excluded either. The NirSoft tools are freeware AND portable, so OP could easily & quickly give the above a try.