When on any comparison of files in Recital, or on the Compare page on the left sidebar, files can be uploaded by dragging and dropping a file on one of the spots for Original or Revised files:

File Upload UI

Files can also be uploaded by pressing the Upload file button. If a file is already selected, it can be replaced by dropping a file on its spot — or by pressing X to clear the file, and then using the Upload file button.

Files uploaded for comparison are also stored and scanned as part of Recital's automatic version history.