I don't see a way to do this either upon pressing on the file to open it. One workaround may be to use the Share option once the text file is opened in the default app. In my case that was the Hancom Office. Once the text file is opened in Hancom Office, click on the three dots in the upper right corner, then choose Share. That will open up options for which app you'd like to share with that in theory can open up and read text files. In my case I have OneNote installed, so could choose that or a few other apps that can receive/read text files.
So, if you have a desired app that you want to use for text files and that can search within files, try installing that and then see if the Share option will let you send the text file to your desired app.
As we discussed above, I don't know why Samsung appears to have removed the option for "Always or Just once". Don't understand that logic at all, and in fact that feature is one that used to give TouchWiz and advantage over iOS.