If I remember correctly, .m4v is a special extension from the MP4 container that Apple uses to allow certain atoms, streams, and metadata to be used for their own video files that they sell on the iTunes Store.
In other words, the .m4v extension is kinda propriety to Apple products, and so not all other non-Apple devices will be able to read the extension.
Another thing would be that the device that you're using hasn't been updated to recognize the .m4v extension yet.