I suggest a two step submission form as mentioned by MrBen.
In the second step, provide links (and if required instructions) to a couple of the easiest, free on-line large file sending services. There is a list of some of them
here (make sure you check the comments out as there are many more shown there).
Some are very simple - it's just a case of finding the one or two which offer the right balance of ease-of-use, free non-subscription use, large file allowance, etc.