Fix1: Package upload using Python3 was still broken:

Loading file contents using 'r' does not work in Python 3
and results in an encoding error. Replaced with 'rb'.

Fix2: Be more robust when MIME types are detected:
Treat 'application/x-gzip' as 'application/gzip'.
(This will also allow to use osmclient on OS X)

Change-Id: I430b2fae2c275803120a6815d01bb26bff26ebdc
Signed-off-by: peusterm <manuel.peuster@upb.de>