o1-file marks a HTML tag to contain a file. The attribute
data-o1-path selects the file.
These are used for identifying HTML elements as files. They will turn the HTML element into a handle for the file. It will be selectable and droppable, you will be able to open the contextmenu on it, use cut/copy/paste, etc.
Note that the entire document (page) is recognized as a file WITHOUT having this class and attribute specified. The path of this file matches that in the location bar.