Consider:

  model = new QFileSystemModel();
  model->setFilter(QDir::AllEntries | QDir::NoDotAndDotDot);
  model->setNameFilters(QStringList() << "*.tiff" << "*.cbf");
  model->setNameFilterDisables(false);
  model->setIconProvider(new ThumbnailProvider);
  model->setRootPath(...);

where ThumbnailProvider implements QFileIconProvider::icon(const QFileInfo& info).

Due to the filter settings, I expected that ThumbnailProvider::icon() would get requests for "tiff" and "cbf" files only, but instead I see requests for icons for all files in the root path of the model.

While this is not incorrect per se, performance would probably improve if no icon for filtered files would be requested/created.