* Keep all uploaded zip content accessible

iterdir() is platform dependent, that is the order of the returned items
may be different on different platforms. In cases where a zip file
contains multiple base_file candidates it will be overridden by the last
one found (which varies on different platforms).

Also, different files with the same extension (file1.csv, file2.csv) will not
be accessible from file_paths as they get overridden, too.

The fix enumerates all files to make them accessible from file_paths.

* Sorts files during unzip

Ensures that unpacked content is sorted before getting handled

* Resolve minor issues

* Ensure index on extensions found multiple times