One supposes that if the file extension is not trustworthy, one can taste file like the file command, and use libraries like the gzip libraries to handle gzipped files as a stream.  There are so many others: zip files could be treated like directories and all the files in them that match the glob could be searched, and then there is bzip2, 7zip, ....  It becomes a popularity contest!  One can do all this with shell scripting, and leave poor old grep out of it!