This is due to the fact that maya's default settings for Mental Ray are 16bit which is not supported by the IFF file formate. Just change the file formate to an (EXR,TIF,or any format that supports 16bit) or you can change the formate to a 8bit RGBA.
So the solution is to select 'TIFF' or 'OpenEXR' from the "Image format" drop down menu in render globals
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Last edited by mythandra; 17-05-2016 at 08:28 AM.