fix(files): Allow downloading multiple nodes not from same base

fix(files): Allow downloading multiple nodes not from same base

When downloading files in e.g. the *favorites* or *recent* view,
then the nodes are not always share the same parent folder
and we can not use the current directory as it is probably just a
virtual one.

So we calculate the longest common base and use that as the directory
for the download endpoint.

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>