Hmm, definitely seems buggy, but not much we can do about it as far as I'm aware. It comes down to how the forum system handles attachments. We've configured it to allow up to 5MB, and it should be allowing up to that. In the case of high resolution images, sometimes it's the *resolution* and not the file size that can cause issues. But in the case of a movie it shouldn't be a factor.
The version of the forum system we're using is still in release candidate stage, so there will be bug fixes to come. Development seems a bit slow, but fortunately this is the first time we've seen this issue so it at least does not seem common. I'm sorry it's affected you though.
In general we have file attachment support here to allow people to easily share images and small files like TGDs and occasional other things. We allow movie uploads, but obviously the file size limits significantly restrict that since most videos are larger than 5MB. I'd suggest if possible you just maintain a YouTube or Vimeo account to upload videos to you and link to them here. That will generally be more flexible, and also easier for at least some users since they can just click-and-play rather than having to download a video and watch locally on their system (especially important for e.g. mobile users).
- Oshyan