Fix file extension detection for files with multiple dot separators

[nerlihmax]

Hi!

I have issues with the flag --add-encoding-exts=js,html,css when uploads js static on production buckets. My files have names like widget-no-csr.oWiXNN3Poy.js or widget-no-csr.oWiXNN3Poy.js.gz, where is more than one dot symbol separator. Because

s3cmd/S3/S3.py

Line 736 in 10aea09

parts = filename.rsplit('.',2)

there is file extension oWiXNN3Poy for first file and js for second. That looks like a bug. I fixed that, but I think it could be kind of dangerous because it breaks backward compability.