@leocg
We seem to be getting nowhere with this.
Is there any reason why Opera is 'designed' to prevent users from having the convenience of opening linked files directly in their preferred application? (Especially if Opera can't 'handle' those files.) From your earlier comments, it seems that other users also find Opera's current behaviour undesirable in this regard.

Maybe I misunderstood this comment from you:
"As (almost) any other program would do, Opera opens the files that it can handle when you open those files from inside Opera."
From the preceding comment ("You don't need to open another browser to export a citation, it works in Opera."), I thought you were implying that citations (RIS format, etc.) are files that Opera can handle. (It cannot!) Maybe you were saying Opera doesn't open citations within the browser, because it cannot 'handle' them?
And when you say Opera can "export" a citation, what you really meant was that Opera can save these citations as individual files to disk — not directly export them to a reference management application?

Also, was there a misprint in your last response? Your two statements seem contradictory
"Because it doesn't have an option to directly open downloaded files in a specific program"
and
"Exactly because, as I said, they have the option to directly/automatically open downloaded files in a specific program."
Or is "they" referring to other browsers (not Opera)? In which case I'm not sure when you said that before.

—DIV

@leocg
Semantics
OK, I don't really want to get caught up in a debate over semantics. I, personally. think of "exporting" a citation as pulling the information out of the online database and into my reference manager (which happens to be EndNote). It's true that the citations can (typically) also be saved to disk as text files, which I think of as "saving to disk". If you want to use other terminology, then feel free, but at least we now understand what's involved.

Substance
You write that
"As (almost) any other program would do, Opera opens the files that it can handle when you open those files from inside Opera."
This statement is wrong on three counts:

• Opera does not attempt to open the citations (or provide this option); rather, it only provides the ability to save them to disk.
• Opera does not have the ability to "handle" citations saved in a structured file format; that is like saying that Notepad can "handle" XML, because Notepad can display an XML file as pure text. (Sometimes we might want to view the XML file as pure text, but nobody should call Notepad an XML viewer!)
• This behaviour is not what (almost) any other comparable application would do. I have already stated that Internet Explorer, SeaMonkey, Edge, and Firefox all handle citations properly, while Opera is the odd one out.
  Moreover, even if (hypothetically) some other applications had poor functionality, is Opera's philosophy to try to be "as bad as our competitors"???
  By the way, all of those other browsers do additionally offer the option for the user to save the citation file to disk, which is occasionally (not often) desired.

Seriously, even if there are some users out their who (weirdly) want Opera to behave like this, does that mean that it must be foisted on everyone with no other option? (Other than the increasingly attractive option of abandoning Opera altogether?)
—DIV

@leocg
Incidentally, if you wish to broaden this to a question of "Why can't I open ______ directly in my desired application?", then you could also review the apparent impossibility of opening a file (e.g. a PDF document) in one's preferred viewer directly after downloading it. The file is listed in Opera's download manager, but Opera insists that only it is allowed to open it. If I want to open it in my preferred application (e.g. a PDF viewer/editor), then I am forced by Opera to open the folder into which the file was saved, and then open the file from there. [Or else manually launch the preferred application and then either manually navigate to the saved location (or possibly copy the location from Opera's download manager).]
—DIV

@leocg
I suppose that's 'just' another way of describing it.
Although I wouldn't personally include "just", which seems a bit dismissive, whereas I would argue this is a serious issue. Who wants to run open a second browser every time they want to export a citation?
(Sorry if there are lots of such requests already made, but I did search the Opera forums for any previous posts regarding citations/references and so on.)

Is there a way of doing that already?
If not, is there a compelling reason why it's not available? Or is work on the software already in progress to address the general request?

—DIV

Another example is at https://projecteuclid.org/euclid.aoms/1177705791#export
See "Export citation" button at bottom of web page.

Internet Explorer (version 11.0.9600.19846) and Edge (version 86.0.622.51 x94) both give the user an option to either directly open the citation (e.g. *.ris file) or to save it to disk. The user could open it in their desired application, but usually they'd want to open it in their reference manager software so that it can be directly imported into their database.

I'm fairly sure that Firefox does the same thing as SeaMonkey too.

I never use Google's Chrome, but Edge is a "chromium-based browser", and the functionality works appropriately there.

Opera is the only browser I can name that fails on this. (Please let me know if there's an obscure setting somewhere that will fix it.)

—DIV

Opera should be able to support the export of citations from sites that host academic articles. Such citations are commonly in RIS format, although others exist (e.g. ENW, JSON, NBIB).

For example, for this article
https://dx.doi.org/10.1177%2F159101991001600204
Cite > Download citation
or
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3277972/
Download as RIS

Opera only gives the option to save the citation as a file. Other browsers, including SeaMonkey, have native support for opening the file directly in the user's preferred bibliographic database application, such as EndNote. (Sometimes this is referred to as "direct export".)

Opera is barely usable by academic researchers without this capability.

—DIV