Here's a weird one. I was trying to attach what I thought was a 1.6 MB .GIF file to a post on the
"Lovely Lilith" thread. The .GIF came from her Clips4Sale store, and was a sample of one of her videos.
Every time I would click the "Post" button, I would instantly be taken to a "Create a new topic" page, as if I was trying to create an entirely new thread. I did this about four or five times, and then decided to take a closer look at the .GIF in question in Preview. I opened the file, and then tried to "Export" it. When I did that, the 1.6 MB .GIF expands to a 17 MB file (still supposed a .GIF though).
Normally, if you try to attach a file of any sort that is larger than ~3 MB, you'd get an error message. In this case, I am being taken to a "Create a new topic" page instead.
Even stranger, while I am making this post, I tried to do it again so that I could screenshot it. Instead of taking me to the "Create a new topic" page, now the "upload" just seems to go on forever, until I close the tab.
I'm on OS X 10.8.5, and I'm posting from Firefox v.48.0.2.
This isn't a worthwhile answer, but sometimes there's just something wrong with a file that makes it really confuse the forum software during an upload. It may be that the file was corrupt, or it may have been compressed using a protocol that your Mac and the forum software don't know how to handle. You eventually got the image to load, so we'll just shrug and be thankful for that. —gonZo