So by the very nature of it if your decompressor is non-zero in length, you have to find some efficiency/tricks to make the file smaller.
Realistically the overhead on the decompressor will be 10 KB and that is before any kind of actual logic, so you are looking at shaving a minimum of 50 KB off of the original file (which is completely random).
So if, for example, 2% of files can actually be reduced in size (I don't know what the actual number is, and if it is even computable) that's still positive EV in a $100 vs $5000 bet.
That is, if the file was truly random, it would have a chance of 1/(2^X) of being all 0, where X is the size of the file. But since Mike would reject that file, the chance is actually 0.
Same for all files with easily exploitable patterns - for example, I am sure that Goldman checked that the file could not be compressed with gzip before sending it.
So the EV is probably not positive, even if there is a small chance that a random file of size X can be compressed.