What format is a Storyteller readaloud, and can Calibre play the synced highlighting?

The download button next to the readaloud icon gives me a plain .epub, which surprised me since the settings mention a "readaloud" suffix. I want to know whether that downloaded file carries the alignment, and whether Calibre can actually play it with text highlighting or if I need a plugin.

Resolution: A Storyteller readaloud is an ordinary EPUB 3 file that uses the open Media Overlay specification — the audio and the alignment metadata are baked into the .epub itself, which is why several third-party readers can open it. There is no separate extension: the "readaloud" suffix setting only controls how files are named inside Storyteller's own folder layout (files stored under assets skip the suffix entirely because Storyteller owns that directory), and it is stripped on export. Calibre needs no plugin — import the .epub and open it in Calibre's viewer, then switch the viewer into read-aloud mode, where you get play/pause and speed control, with navigation by double-clicking the text you want to jump to. Its media-overlay support is basic compared with the Storyteller web reader. One Calibre caveat: import as a new entry (or into a separate library) rather than overwriting, because Calibre will not hold the original and the readaloud as two files under one entry and you can clobber your source book.