Update Images Changed on Disk

This is for #6356. Continued work @couzteau 's work (pull requests #6443, #7952) to only check that the file was changed on disk and refresh and not close/reopen the file unnecessarily.
`_updateFileModificationTime` returns a `Deferred`, but it is ignored by both places that call this function. Is it not necessary, or should callers be waiting for it?