You can't do this (easily) in Ninox due to how RTF files are structured/encoded.

The best/easiest way to go about this is to automate using CloudConvert with make.com (its possible with both of their free tiers)

The make scenario would:

• Receive a webhook (http request) from Ninox containing the record ID and file name
• Download the RTF file from Ninox
• Cloudconvert module - upload and convert to RTF - this module will then return the text file
• Upload the text file to Ninox OR post the contents back to the record in Ninox as plain text

Hope this helps!