This question tends to come up quite often lately. As it has already been pointed out to you it's basically the knapsack problem, which is known to be generally a "hard" problem. However a practical answer may depend on the actual average file sixe: if you only have files whose size is about say 1Mb or less, or at least you have a good wealth of such files along with potentially larger ones, then you may be content with a suboptimal solution given by filling up the space with as many of those files as possible.

As a side note, outside of France (for what I know) Mo is spelled Mb...