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 file containing the bucket and the bucketId.
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.
Note that when no ancestor exists that fulfills this contract, the entire document (page) will be recognized as the drop target, specified by the path in the location bar.
The bucketId will be taken from the data-o1-placeholder attribute of the
current DOM node.