No satisfactory explanation has been given by Opera for this deficiency

It seems to be the default for (almost) all major browsers nowadays. Also it may not be very convenient to see a file opening hours later while you are doing something else, a scenario that can happen if the download runs in a slow connection.

So, Opera and other browsers are trying to regulate stupid??? Very, very frustrating and is why I usually have more than one browser open. If Opera would allow folks to turn ON the feature (versus not allowing it at all), I might be able to stick with just one.

I also miss this vital feature. No satisfactory explanation has been given by Opera for this deficiency (other than the standard "for security reasons" excuse).
There is one extension that accomplishes half of what you're asking, namely:
https://addons.opera.com/en/extensions/details/open-with-application/?display=en
It saves the file to a temporary location and then opens it. In my case, I chose the Desktop as the temporary location, so every now and then I get a folder called somethi8ng like "temporary download folder" on my Desktop. If you click a file but then decide not to open it, the extension deletes the file from your hard drive again. This can be bothersome if you think you've downloaded a file, and then discover that the file isn't there where you've seen it just a moment ago, and you have to figure out where to download it from again (because unlike in Opera 12, the current Opera doesn't keep a history of all visited pages).

Thanks for that suggestion. I'll check it out.

No satisfactory explanation has been given by Opera for this deficiency

It seems to be the default for (almost) all major browsers nowadays. Also it may not be very convenient to see a file opening hours later while you are doing something else, a scenario that can happen if the download runs in a slow connection.

There is one extension that accomplishes half of what you're asking, namely:
https://addons.opera.com/en/extensions/details/open-with-application/?display=en

It saves the file to a temporary location and then opens it. In my case, I chose the Desktop as the temporary location, so every now and then I get a folder called somethi8ng like "temporary download folder" on my Desktop. If you click a file but then decide not to open it, the extension deletes the file from your hard drive again. This can be bothersome if you think you've downloaded a file, and then discover that the file isn't there where you've seen it just a moment ago, and you have to figure out where to download it from again (because unlike in Opera 12, the current Opera doesn't keep a history of all visited pages).

Is there a way to accomplish

Nope.

Maybe through an extension.