FileConvert

FileConvert[source->"dest.ext"] converts the contents of source to the format defined by the extension ext and writes the result to the file dest.ext. 

FileConvert[source,"format"] writes the result to the filename defined by source, but with an extension defined by the specified format.

FileConvert[source->"dest.ext",format1->format2] takes the contents of source to be in the specified format "format1".

Please visit the official Wolfram Language Reference for more details and examples on core symbols.