The word miscellaneous means a set of things that is heterogeneous, that is to say, the things in it are of a diverse nature or they relate to a wide variety of subjects. In this case, the file cabinet in question contains all the products of people´s extra work, which means that, though the cabinet may be filed in a certain order, the contents are diverse.
Context clues! Because it they put in the 'extra work' there you can assume that it is made up of different parts because it has many different files. So the last one, made up of different parts.
