When you upload a file of the same name as another file already attached to the post, it should automatically update -- or, at the most, prompt you asking if you want to update the file or upload it as a new file. The "Add Attachment / Update Attachment" dichotomy is very confusing.
It should definitely ask if you want to overwrite/update the previous one.
Well, I dunno, I guess so. I think most users would expect that if they upload an attachment with the same name as one they've already uploaded, it will overwrite.
This would be great as far as making sense, and it would also save disk space on the account or server that the forum is on.
This is something I have noticed a number of times, I noticed it most when making a new thread adding an attachment with the same name you get this notice.
"This post already contains an attachment with the same name. Please rename the file and upload it again. Alternatively you may click "Update Attachment"."
But there is no Update Attachment link/button only "Remove" "Insert Into Post" So I'm all for this.
Then you would probably class this as a bug in that the 'Update' button doesn't appear.