MSWMM is the project file of Windows Movie Maker, not a real video format, which is only compatible with Windows Movie Maker, so that it is impossible to play MSWMM files on fashionable portable devices and players or directly convert MSWMM to more popular formats like AVI, MP4, MOV, etc.
There is
an article which informs us:
Part 1 Save MSWMM to WMV or MP4Run Windows Movie Maker and drag the MSWMM that you want to convert to WMM to add MSWMM file. Click File menu to select Save Movie File... to save it as WMV or MP4 and also choose the location on your computer.Part 2 Convert MSWMM to AVI, MP4, etc