xml validation, removed redundant boxing, unknown files fail validation, caught a inverse logic error

src/core/DataFile.cpp

@@ -202,7 +202,7 @@ bool DataFile::validate( QString extension )
 		}
 		break;
 	default:
-		return true;
+		return false;
 	}
 	return false;
 }

src/core/PresetPreviewPlayHandle.cpp

@@ -151,15 +151,13 @@ PresetPreviewPlayHandle::PresetPreviewPlayHandle( const QString & _preset_file,
 			dataFileCreated = true;
 		}
 
-		DataFile data(*dataFile);
-
 		// vestige previews are bug prone; fallback on 3xosc with volume of 0
 		// without an instrument in preview track, it will segfault
-		if(data.content().elementsByTagName( "vestige" ).length() == 0 )
+		if(dataFile->content().elementsByTagName( "vestige" ).length() == 0 )
 		{
 			s_previewTC->previewInstrumentTrack()->
 					loadTrackSpecificSettings(
-						data.content().firstChild().toElement() );
+						dataFile->content().firstChild().toElement() );
 		}
 		else
 		{

src/core/Song.cpp

@@ -942,7 +942,7 @@ void Song::loadProject( const QString & fileName )
 	DataFile dataFile( m_fileName );
 	// if file could not be opened, head-node is null and we create
 	// new project
-	if( !dataFile.validate( _file_name.right(_file_name.lastIndexOf(".") ) ) )
+	if( dataFile.validate( _file_name.right(_file_name.lastIndexOf(".") ) ) )
 	{
 		return;
 	}

src/gui/FileBrowser.cpp

@@ -466,7 +466,8 @@ void FileBrowserTreeWidget::mousePressEvent(QMouseEvent * me )
 			if( !dataFile.validate( f->extension() ) )
 			{
 				QMessageBox::warning( 0, tr ( "Error" ),
-					f->fullName() + " " + tr( "does not appear to be a valid") + " " + f->extension(),
+					f->fullName() + " " + tr( "does not appear to be a valid" ) + " " + f->extension() +
+									  " " + tr( "file" ),
 					QMessageBox::Ok, QMessageBox::NoButton );
 				m_pphMutex.unlock();
 				return;