Previously we used uniqid() here.
However this means that the behaviour is not reproducable, so
when uploading files into a "empty" folder, the folders name is
different.

If there would be a md5() hash collition, the deduplicate check
will spot this and append an index later, so this should not be
a problem.

Fix #6722

Signed-off-by: Stephan Peijnik <speijnik@anexia-it.com>

As constants not defined within a class cannot be automatically found by the
autoloader moving those constants into a class makes them accessible to
code which uses them.

Signed-off-by: Stephan Peijnik <speijnik@anexia-it.com>

ref #10392

Fix #9928