added

This was possible with Textastic 5.3.1 and earlier, but since I released Textastic 6 as a new app, it doesn't allow you to download versions earlier than Textastic 6.0, so it only works with iOS 9 and later.

I'm pretty sure this didn't happen in earlier macOS versions, but I can see this problem here, too.

You can already do that: open the JavaScript file and show the web preview. Then tap on the address bar, enter the path of the HTML file and press Enter on the virtual keyboard.

From now on, Textastic will show the HTML file when you open the preview of the JavaScript file.

Also, which version of the app do you use?

Thanks for letting me know!

On which iPhone model does this happen? Did you see any pattern when this happens? Which buttons exactly (below the file list? below the editor? below the "Local Files"/"iCloud" selection?)

I use the app myself every day and haven't notice this so far, so any additional info helps.

Do you mean as a sort order of downloaded files? How do you plan to use this feature?

Wouldn't it be easier to download files from different hosts to separate folders?

I'll consider it. I thought the list wouldn't be useful without seeing the matches in the file.