If your system where you procedure the .swf file is already infected, it is possible the .swf will include a Trojan without you knowing about it. It is also possible someone hacks your website and adds a Trojan to the .swf file, or replaces it with an infected one. Both pretty unlikely, but not impossible.
That said, if the forum allows you to include a link to an external .swf file, then that’s fine. People will have to make there own decision to trust a link or not.