can be done easily by piping your file or directory search to the Select-String cmdlet (or sls shorthand):

ls . *.txt -r | sls -pattern "My fat string example"

Specifying (or excluding via -exclude) multiple file types:

ls . -include *.txt, *.resource -r | sls -pattern "My fat string example"

The -Include operator might be to slow when there are to many files to search, thus:

ls . -r | where { $_.extension -in ".txt", ".resource" } | sls -pattern "my fat string example"