As of March 31, 2021, uploading flash files is no longer possible. See forum #183397 for details.
As of March 2021, Danbooru is set up to automatically load all Flash posts using Ruffle.js, which uses WebAssembly to emulate Flash in modern browsers as a part of the broader Ruffle Project. While this means you can view Flash posts without falling back on an outdated browser that's old enough to still support the original Flash Player, please keep in mind that Ruffle's core emulation of Flash is still in an alpha state and it's less likely to work with files that are newer and/or more complex. See ruffle compatible for posts that have been manually confirmed to work with the version of Ruffle.js that Danbooru is currently using.