Fix broken MIDI import

Fix a regression in 3e538d5 (#4401) that
readAllData doesn't read the file from the beginning

Author: PhysSong

include/ImportFilter.h

@@ -80,6 +80,7 @@ class EXPORT ImportFilter : public Plugin
 
 	inline QByteArray readAllData()
 	{
+		m_file.seek(0);
 		return m_file.readAll();
 	}