I'm also noticing that when I click "Remove" under "Current Attachments" on the popup to get rid of the weird pop-out preview thing, they disappear, but not when I leave if there. Am I missing something there?
...I think I might know what's going on. The file is most likely in the temporary cache, as you attached it to a post but never actually saved it. Did you upload the file while not on the exact same post editing screen? If you clicked anything in the post editing page BEFORE uploading the file, I think it might have invalidated the pop-up upload window and anything that you did in it. I believe I cannot view it because files are only able to be viewed by the poster until they are made public by saving for security reasons.
I put them up on March 15. I checked the next morning to see if they were still there and they were; then I checked about a half hour ago and they've disappeared again.
Yeah, there is no reason that should happen. I've seen it occasionally, but I don't think I've seen them disappear a day or two later out of the blue, so my guess is something happened during the upload.
I'm pretty sure they uploaded normally the first time, though . . . well, I replaced them just now, and they show up in the post, so I guess that's a good thing. There's no obvious reason they should get de-attached from the forum or anything, right?
I'm not sure why your blog's attachments are disappearing. One guess is that you're opening the attachment window, clicking the preview button in the main window, and then uploading the file.