class="o1-placeholder" data-o1-placeholder="1"
A placeholder is a frontend concept that maps a DOM node to a bucket on the filesystem. When a file is dropped in a placeholder it will be routed in the corresponding bucket. For that, the system needs to identify the bucket name and the file containing it.
To identify the file, Boomla will look for the closest DOM node that defines
the o1-file class, and will use the data-o1-path attribute of that DOM node.
When no such ancestor exists, the entire document (page) will be recognized as the drop target, identified by the document URL.
The bucket name will be taken from the data-o1-placeholder attribute of the
current DOM node having the o1-placeholder class.