I think the wording in http://www.ldraw.org/article/218.html#extension is about as correct as it can get. All three extensions are legal, all three are extant in the wild, and they're interchangable.
The very existence of .mpd is unfortunate, but even more unfortunate is that .ldr was not explicitly called out as being valid for both MPD and non-MPD files from the very beginning. That is the case de facto, but stating it explicitly would have eliminated some user confusion, provided stronger branding, and made it more obvious that MPD is an inseperable part of the LDraw language. No user should ever need to care how many models his file contains.
Bricksmith has been saving all files as .ldr regardless of content since its inception in 2005.
Allen
The very existence of .mpd is unfortunate, but even more unfortunate is that .ldr was not explicitly called out as being valid for both MPD and non-MPD files from the very beginning. That is the case de facto, but stating it explicitly would have eliminated some user confusion, provided stronger branding, and made it more obvious that MPD is an inseperable part of the LDraw language. No user should ever need to care how many models his file contains.
Bricksmith has been saving all files as .ldr regardless of content since its inception in 2005.
Allen