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!