We could make the full UF2 public at some point.
For now, in the IDE go to UTILITES->DOWNLOAD FIRMWARE and hover over the button for instructions on getting MicroPython back on the device. After you install the MicroPython UF2 to Thumby, connect to the IDE using the connect button and then click the format button in the filesystem panel.
This will put Thumby into the default state it shipped to you with - although some files may have been updated or added since then.