Files data directly displayed in browser instead of downloading the file

Try to set "Enable download accelerator" to "None" under Control Panel > "Files" > "Misc options".

Not much difference it seems

https://www.files.olokos.pl/wl/?id=MCeDihTf5XvHq8RCqbh81rlRoAACLIL8

I've had it set on XSendfile before

Have you edited any FileRun files at all?

Are you using a reverse proxy?

I haven't done any modifications to filerun apart from those within the filerun webpanel itself.

I am not using reverse proxy for filerun at all

Here's my apache2 virtualhost for filerun

https://termbin.com/w6g0

I have just recently added the filesmatch .php and it makes absolutely no difference for this issue, whether I have it or not - apart from website loading speeds, those have improved, but please do feel free to correct any mistakes you would spot here, it would be much appreciated.

The HTTP headers sent out by your web server does not match what FileRun sends. For example "Content-Type" is missing and "Content-Disposition" is overwritten with modified values. I can tell you this for a fact, but I wouldn't know what exactly is causing this in your setup.

Could it be that this is because I am using HTTP/2 protocol?

I don't know.

If you can't find what is causing this, you can also enable the option "Prevent the browser from opening the file." in the web link's advanced settings.

I disabled HTTP/2 protocol, but it makes no difference :(

I cannot find "Prevent the browser from opening the file." option anywhere on top of that :/

Any suggestions where I should go looking for something that is causing this?

I cannot find "Prevent the browser from opening the file." option anywhere on top of that :/

In the "Share" > "Web Link" panel, click "Advanced".

Any suggestions where I should go looking for something that is causing this?

I would review the way PHP-FPM is installed in Apache, perhaps trying a reinstall.