Вы можете использовать:
ListWidgetItem::setData(int role,Qvariant data);
для хранения filePath;
как это:
QString fileName = "hello.txt";
QString filePath = "/home/user/hello.txt";
//Adding to listWidget
QListWidgetItem *item =new QListWidgetItem();
item->setText(fileName);
item->setData(1,filePath);//here role is set as 1
listWidget->addItem(item);
//For taking back that filePath
QString filePath = listWidget->currentItem()->data(1); //note::: role equals 1