Does this matlab doc help you?
http://www.mathworks.com/help/techdoc/ref/save.html[
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From the documentation
filename
:
Name of a file. If you do not specify filename, the save function saves to a file named matlab.mat. If filename does not include an extension and the value of format is -mat (the default), MATLAB appends .mat. If filename does not include a full path, MATLAB saves in the current folder. You must have permission to write to the file.
Default: 'matlab.mat'