Chunking NG: Assemble in natural sort order of files

For https://github.com/owncloud/client/pull/5476

Before this, the assembly could be bogusly in the order 0,1,10,11,2,3 etc.

As per the spec "The name of every chunk should be its chunk number."
https://github.com/cernbox/smashbox/blob/master/protocol/chunking.md



Signed-off-by: Morris Jobke <hey@morrisjobke.de>