How to control which file extensions are registered with this dll?

By default (and during COM registration) the QuickTime Source is registered with the following extensions: .mov; .qt; .mp4; .m4v; .264; .amr; .3gp; .3g2; .3gp2; .aif; .aiff; .flc; .cif;. dv; .dif.

But you can register only specific extensions or add the new one via QTRegisterExtensions / QTUnregisterExtensions exported function call:

QTRegisterExtensions( ".mov\0.qt\0" );

or RunDLL_QTRegisterExtensions / RunDLL_QTUnregisterExtensions calls (for register via RunDLL32.exe).

e.g. for register only MOV and QT extension, you need to run the following script (just after install):

rundll32.exe QTSourcePXT.dll RunDLL_QTUnregisterExtensions rundll32.exe QTSourcePXT.dll RunDLL_QTRegisterExtensions .mov;.qt

Note: The extension should be supported by QuickTime runtime (e.g. can be played via QuickTime Player), e.g. you can't rename .MOV to .ABC and register the .ABC extension for QTSource filter.