Use a key that's distributed and at least partially redundant. For example, break the key into 20 sections, and allow decryption with a minimum of at least 11 of those sections present.
Distribute the key sections to geographically diverse, trusted people, in different countries with different governments, with the instructions to keep them somewhere safe, and on a certain date (ie: Jan 1, 2020) publish them online in a known location.
Sure, some people might be jerks, or accidentally publish ahead of time (or not at all), but assuming that (in this case) 55% of the keys are available, the file can be unlocked. Of course, you could change the number of key sections required based on how critical secrecy is vs. security, etc.