Actually, such checkboxes are valid according to the RTF specification, but the Form Fields feature is not supported by RadRichTextBox at this point. This is in our backlog already and you can vote for its implementation in our public portal: Implement support for Form Fields
As to attachments, our public forums do not allow attaching files other than images and this is not a recent change. I checked your support history and it seems you've previously contacted us through support tickets which allowed you to attach files.
Let me know if I can help you with anything else.
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