Filtering in QFileDialog

I believe what you can do is:

1. Create a custom proxy model. You can use QSortFilterProxyModel as a base class for your model;
2. In the proxy model override the filterAcceptsRow method and return false for files which have the ".backup." word in their names;
3. Set new proxy model to the file dialog: QFileDialog::setProxyModel;

Below is an example:

Proxy model:

class FileFilterProxyModel : public QSortFilterProxyModel
{
protected:
    virtual bool filterAcceptsRow(int source_row, const QModelIndex& source_parent) const;
};

bool FileFilterProxyModel::filterAcceptsRow(int sourceRow, const QModelIndex &sourceParent) const
{
    QModelIndex index0 = sourceModel()->index(sourceRow, 0, sourceParent);
    QFileSystemModel* fileModel = qobject_cast<QFileSystemModel*>(sourceModel());
    return fileModel->fileName(index0).indexOf(".backup.") < 0;
    // uncomment to call the default implementation
    //return QSortFilterProxyModel::filterAcceptsRow(sourceRow, sourceParent);
}

dialog was created this way:

QFileDialog dialog;
dialog.setOption(QFileDialog::DontUseNativeDialog);
dialog.setProxyModel(new FileFilterProxyModel);
dialog.setNameFilter("XML (*.xml)");
dialog.exec();

The proxy model is supported by non-native file dialogs only.

The solution of @serge_gubenko is working well. Create your own ProxyModel by inheriting from the QSortFilterProxyModel.

class FileFilterProxyModel : public QSortFilterProxyModel
{
protected:
    virtual bool filterAcceptsRow(int source_row, const QModelIndex& source_parent) const;
};

bool FileFilterProxyModel::filterAcceptsRow(int sourceRow, const QModelIndex &sourceParent) const
{
    // Your custom acceptance condition
    return true;
}

Just make sure to set DontUseNativeDialog before setting the Proxy model (Edit: @serge_gubenkos answer does that now). Native dialogs do not support custom ProxyModels.

QFileDialog dialog;
dialog.setOption(QFileDialog::DontUseNativeDialog);
dialog.setProxyModel(new FileFilterProxyModel);
dialog.setNameFilter("XML (*.xml)");
dialog.exec();

It took quite some time for me to find this out. This was written here