Different behavior for drag-and-drop of example chapter files

Maybe #3232 (closed) meant something similar, maybe not.

When example-chapters-1.xml is drag-and-droped as a source file:

The file '.../example-chapters-1.xml' contains chapters. These aren't treated like other source files in MKVToolNix. [...]

When example-chapters-2.xml is drag-and-droped as a source file:

The file 'example-chapters-2.xml' was not recognized as a supported format (exit code: 0).