add application/gzip mime type config for .gz files if they are requested directly, overriding the common /etc/mime.types entry application/x-gzip
if a .gz file is uploaded, calculate the hash based on the uncompressed contents

otherwise there would be an ambiguity between application/octet-stream files with gzip on-disk compression and application/gzip files with no compression, and they would appear to have different hashes, which would break replication