I believe you :)

However the issue is 99,999% related to wrong MIME detection in your server. In other words, the server cannot properly detect the file being uploaded is a PDF and as such does not pass on that info (for a valid file upload) to the PHP library that K2 users for file uploads. As such the uploader is renaming the file to a safe .txt extension.

Until the matter is addressed differently to cover cases where MIME detection is not done properly or at least not in a way that is compatible with the upload library K2 uses, you can use the "browser server" option, upload the file through that modal and then select to insert it.

It depends on your server setup. What's the underlying webserver? Apache, Nginx, IIS?

On a stock (current) cPanel server, there are no MIME setup issues with Apache.

Unless it's some old cPanel version with EA3 and a custom Apache build. More info would be helpful.

Are you using the stock EasyApache4 setup? Or did you reconfigure it from scratch?

Alternatively, try this EA4 profile from Engintron: raw.githubusercontent.com/engintron/engintron/master/cpanel/EasyApache_Profiles/EA4/Engintron_EA4_2019_v1.json

Try commenting out this line: github.com/getk2/k2/blob/master/administrator/components/com_k2/models/item.php#L439

Let me know.