Multiple attachments support

[Ribas160]

PrivateBin web interface supports multiple attachments since release 1.7.7

It was implemened through sending arrays instead if strings in attachment and attachment_name parameters. This approach allowed to not change the message structure and insure backward compatibility. Since the changes only concern the client side, I suggest to add support for multiple attachments to this CLI client.

If you don't mind, I can do a PR and would appreciate your review.

[mxmehl]

With a rather recent PrivateBin version, pbincli fails with the following error when fetching a pad with one attachment:

Paste received! Decoding…
Found text in paste. Saving it to paste-abc123.txt
Traceback (most recent call last):
  File "/Users/maxmehl/Library/Caches/pypoetry/virtualenvs/project1-AV6d_t8C-py3.14/bin/pbincli", line 8, in <module>
    sys.exit(main())
             ~~~~^^
  File "/Users/maxmehl/Library/Caches/pypoetry/virtualenvs/project1-AV6d_t8C-py3.14/lib/python3.14/site-packages/pbincli/cli.py", line 210, in main
    args.func(args, api_client, settings=CONFIG)
    ~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/maxmehl/Library/Caches/pypoetry/virtualenvs/project1-AV6d_t8C-py3.14/lib/python3.14/site-packages/pbincli/actions.py", line 228, in get
    attachment, attachment_name = paste.getAttachment()
                                  ~~~~~~~~~~~~~~~~~~~^^
  File "/Users/maxmehl/Library/Caches/pypoetry/virtualenvs/project1-AV6d_t8C-py3.14/lib/python3.14/site-packages/pbincli/format.py", line 93, in getAttachment
    return [b64decode(self._attachment.split(',', 1)[1]), self._attachment_name] \
                      ^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'list' object has no attribute 'split'

I'd assume the linked MR would fix this issue, which would be great!