Do not hide the progress bar while the chunked upload is being assembled

Large files are not uploaded in a single operation, but uploaded in
several chunks; once all the chunks are uploaded then the server needs
to assemble them to get the final file.

Before, once the chunks were uploaded the progress bar was hidden.
However, this was confusing for the users, as the file could still need
some time to appear in the file list due to the assembling. Now once all
the chunks are uploaded the text in the progress bar changes to inform
the user that there are still some pending operations, and only when the
file is finally assembled the progress bar is hidden.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>