This issue seems to only affect export to XML. Export to text file defaults to "All Files" in the file selection dialog, where XML defaults to *.xml. Otherwise the source code looks pretty similar for both:
Export_KeePassX_Xml.cpp: bool Export_KeePassX_Xml::exportDatabase(QWidget* GuiParent,IDatabase* database){ db=database; QFile *file=openFile(GuiParent,identifier(),QStringList()<<tr("XML Files (*.xml)") << tr("All Files (*)")); if(!file)return false; Export_Txt.cpp: bool Export_Txt::exportDatabase(QWidget* GuiParent, IDatabase* db){ QFile *file=openFile(GuiParent,identifier(),QStringList()<<tr("All Files (*)") << tr("Text Files (*.txt)")); if(!file)return false; Unfortunately I don't have the Qt skills to debug this further.