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.

[fviard]

Thank you for your PR, indeed the existing code looks very strange, and my best guess is that it was not done on purpose but a confusion with the count argument of the python split.

[fviard]

And btw, don't worry about the automatic test failing, it is not related to your PR :-)

[snosratiershad]

@nerlihmax, @fviard LGTM.

[fviard]

Merged, thank you.
@nerlihmax For info if you do other PR, it would be good to "clean" or "squash" the commits in your PR for them to be clean once merged.
Here I did with a "squash" through "github" but it is always better if the PR can be merged as-is :-)