I fixed the 11 (edit: now 51) swf files that are currently tagged flash_9. All were flagged in the header as using ActionScript 3, but they don't contain any ActionScript 3 code. After I removed that flag, they seem to work fine.
Should I upload them as children of the existing posts, or (with someone's help) as replacements for the same post? I just flipped the flag, they're byte-identical otherwise, so it's probably safe to replace them but I know Danbooru likes to keep old versions.
Ping @fossilnix and @BrokenEagle98 who wondered about this in previous forum posts.
Edit: I tested every SWF file up to post #2963761 and found 51 that could be fixed this way. All of them are now tagged flash_9. The most recent of the 51 is post #1197146, uploaded June 2012, so there may be little point in monitoring new posts.
Updated