Hello,

It seems that you hit a known issue which has already been addressed in the latest version R1 2022 SP1. Here is the feedback item for your reference:

https://feedback.telerik.com/winforms/1552759-richtexteditor-xamlformatprovider-cannot-import-files-in-net-core-applications 

Feel free to upgrade to the latest version and see how it works on your end. Please let me know what is the observed result.

Regards,
Dess | Tech Support Engineer, Principal
Progress Telerik

Hi,

I am really sorry to hear that you are experiencing any difficulties with the Telerik Ui for WinForms suite. However, according to the provided information, it wouldn't be easy to conclude what causes the undesired behavior you are facing.

A sample project, demonstrating the exact undesired behavior that you are facing, may save a lot of days and efforts on both sides. That is why it is always greatly appreciated if you simulate the problematic behavior in a runnable project. Then, we would be able to make an adequate analysis of the precise case and think about a suitable solution. This is the most important thing you can do to clearly demonstrate your issue. Replicating your issue is no doubt the most time consuming part, but it's going to play the biggest role in your success at demonstrating your issue clearly the first time. It often feels easier to copy and paste the relevant portions of your code into the ticket. The problem with this approach is that the pieces have to be reviewed by somebody who doesn't know what the entire project is supposed to look like or accomplish. Thank you for understanding and cooperation.

I am looking forward to your reply.

Regards,
Dess | Tech Support Engineer, Principal
Progress Telerik