Can Desktop Silverlight 5 Play MP4 video of Windows Phone Successfully?

In this Sprint, we have the new feature - uploading video files in the Silverlight Application (SL5) and then we can play the video directly. 

In order to test this feature, I need to get some video files in advance.

I found out a tool that can download a video from YouTube directly and save it as .mp4 file, but after upload, I couldn't play the video successfully in SL5.

I used iPhone 5 to record a video (.mov) and I played the video successfully in SL5.

I found out a tool to convert .mov to .mp4. After upload, I played the video successfully in SL5.

My colleague used Windows Phone to record a video (.mp4). After upload, I couldn't play the video successfully in SL5.

 If I changed .mov/.mp4 above to .mp3, after upload, I played the video successfully in SL5.

Per my understanding, Silverlight 5 should support .mp4 format. From my testing experience, I can’t play .mp4 video from Windows Phone successfully. 

From this article of MSDN, I still can’t find the answer if Desktop Silverlight 5 can play mp4 video of Windows Phone successfully.

If users upload .mp4 video from Windows Phone 7 in our Silverlight 5 on Windows 7 and it doesn't work , I will remind them: the file can be renamed as mp3 and upload it again.  It will work.