Enable "Open in Textastic..." prompt from within Files app for .Rmd files

When viewing a regular Markdown file in the Files app, and after selecting Textastic from the share sheet once (which sets Textastic as the default app to open Markdown files in), an "Open in Textastic..." prompt will appear in the upper right of the file preview in the Files app.

This would be great to enable for .Rmd files (and basically any other text file extension) because it would streamline my workflow of searching for a file in the iOS Spotlight, then being able to click that prompt to open the file quicker than having to go find and open Textastic first, click "Open..." and find the file.

This might be a limitation with iOS Files though, I'm not sure, because the iOS Files app displays text preview of .md files whereas it does not for .Rmd files.