A .ott/.dotx is the built document with one label changed — ODF's
package media type, Word's content type for the document part — so the
picker offers both and the chosen extension decides which exporter runs.
Opening or saving also records the file in a recent list. The name is in
localStorage so the menu draws at once; the handle that reopens it is in
IndexedDB beside it. A file that has moved, or whose permission the
browser drops on reload, is reported and dropped from the list.
