msdxm.ocx

ActiveX control Reference

An .ocx file contains an ActiveX control on Windows. The file is a DLL in the Portable Executable format with COM interfaces. Applications and older web pages embed the control to add user interface elements. The name comes from OLE Custom Controls. Modern browsers do not support ActiveX any more.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.