Have a look here 1.8 API Introduction
In case of files that are uploaded to the CMS, you will want to upload the file (POST /library) and then use POST playlistLibraryAssign call to assign it to a region/playlist.
Regarding Widget API calls, so all widgets are edited by PUT /playlist/widget/{widgetId}
Parameters that you can use to edit existing video are here http://xibo.org.uk/manual-tempel/api/#!/widget/WidgetVideoEdit
I know it might be confusing because it’s POST call in the documentation, while all the parameters are for the edit call really, as you don’t use that call to upload&assign new video to layout, but that’s something we had to do to make the documentation clearer - mostly because the edit (PUT) widget call does not have widget type in the call.