Hmm.

Another option might be a class derived from QFileSystemModel or a QIdentityProxyModel subclass on top of a normal QFileSystemModel.

When an icon for a file (Qt:ecorationRole) is requested, a cache lookup could check if there is already a thumbnail for the given file.
if not, return a standard icon and trigger an asynchronous creation of the thumbail and notify the view when it is ready.

Cheers,
_