Can't preview

[FraidyCat]

Every time I try to preview or post, my browser tells me that it's blocked a file download, and ask me if I want to permit it. If I say yes, posting works, but previewing doesn't, I get thrown back to a list of threads.

I used to be able to post from this (work) computer. I'm not sure if it's me or you that's changed. If it's me, it's likely not to be something I can do anything to fix. It shouldn't be necessary to download anything to allow previewing/posting, what's going on?