I think you can make some argument about why this isn't possible at 50:1 odds.
A plausible "decompressor" is at least, say, 30 or 100 bytes, so the random file needs to have 30 bytes less entropy than you expected, which happens with probability X where X << 1/50. Sum over the whole domain of reasonable decompressors, and you still don't get there.
This argument could do with more rigor, but I think it's correct. Give me 100 million to 1 odds, though, and I'll take my chances trying to brute force a compressor.
A plausible "decompressor" is at least, say, 30 or 100 bytes, so the random file needs to have 30 bytes less entropy than you expected, which happens with probability X where X << 1/50. Sum over the whole domain of reasonable decompressors, and you still don't get there.
This argument could do with more rigor, but I think it's correct. Give me 100 million to 1 odds, though, and I'll take my chances trying to brute force a compressor.