The same question is asked in a support ticket sent to us (ID: 1072217). If you have any further questions on the topic, please consider continuing the conversation there.
For anyone else interested in the same topic I am pasting my answer from the ticket response here:
"RadEditor Web Part is designed to work only with SharePoint's lists and database. Thus, the functionality you are after is not covered by this product.
The case you have can be accomplished by creating a custom Web Part for SharePoint, where you can possibly use the plain RadEditor control. But the logic to load content from external file and write back in external files should be further implemented."
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