How can I edit an existing gliffy attachment to confluence, to change colors for example?

Question

I want to access the source of Gliffy diagrams in Confluence, for example to change one color with another in a large number of diagrams in a number of pages.

I managed to read / update / write / reload a Gliffy diagram, but displays a "Drat, error 500 diagram under construction" error with an invitation to contact my administrator.

What I did: inspired by @Tim in How to download a Confluence page attachment with Python?, I wrote the following:

confluence = Confluence(
    url= CONFLUENCE_URL,
    username=CONFLUENCE_USER_NAME,
    password=CONFLUENCE_PASSWORD)

def test_get_page_content():
    page_id=852512337
    attachments_container = confluence.get_attachments_from_content(page_id=page_id, start=0, limit=500)
    attachments = attachments_container['results']
    for attachment in attachments:
        title = attachment['title']
        fname = attachment['title']
        download_link = confluence.url + attachment['_links']['download']
        r = requests.get(download_link, auth=(confluence.username, confluence.password))
        if r.status_code == 200:
            with open(fname, "wb") as f:
                file_text2 = r.content
                #  print(file_text2)
                # file_text = r.content.decode(encoding='utf-16')
                string_content = ""
                for bits in r.iter_content():
                    try:
                        string_content = string_content + bits.decode()
                    except Exception:
                        pass
                edited = re.sub('ff9900', 'CCCCCC', string_content)
                f.write(edited.encode())
            confluence.attach_file(fname, name=title, content_type=None, page_id=page_id, title=title, space=None, comment=None)

When I look at the file in a text editor, it looks like normal JSON, just like a file exported manually in Gliffy format from the Gliffy diagram.