Files which end in tar.gz are supposed to be gzipped tarballs.

tar = tarball This is only necessary if you have more than one file. For some reason some people tar one file and then gzip it! If you only have one file, then just release it as is.

tar.gz = gzipped tarball This is a compressed tarball. I'm going to type tar xvzf. If it fails, then my first assumption is something went wrong with the download. Only after I download it again (costing you bandwidth) and see exactly the same file size will I assume that it was an error on your part.

Yes, I could run file on every downloaded file to determine what it actually is. However, if you're making code available, why make it difficult to get to?