@ewankeep I think FLAC could have a very long tail because it fills a niche most audio formats don't, and by being lossless it (largely) ensures that there isn't loss with subsequent generations of the files. So it's both more likely to stick around compared to other audio formats, and more useful in that long-term sticking around (which thus may further incline people to keep it alive as a format).
Perhaps an argument could be made for raw PCM though since that'd be easier for future generations to work out decoding thereof from the mere bits, assuming the maximum situation of "we somehow have the raw data of this file, with zero accompanying information or context". It's also a method that was somewhat independently invented a few times IIRC, and in use since some of the absolute earliest digital audio encoding, showing both the (relative) simplicity and the low level of technology potentially required for reproduction. (I mean hell, it was even within our lifetimes that there were digital devices out there that didn't have the horsepower to decode MP3s but could decode WAVs at the same sample rates.)
For images I'd be inclined to somewhat similarly imagine simpler formats to reverse-engineer sans context would be the best bet, although I'm unfamiliar with the nitty gritty of most of 'em. I'd actually imagine SVG files, at least uncompressed ones, would be pretty long-lasting, since it's potentially just plaintext describing geometric math.
