I have noticed that several of the files I have attached to the forums keep disappearing. This is really annoying, especially when it happens to my mods. I am not sure, but I think this issue only happens with the OP of a thread. I have discovered a work around for this issue but I would have to edit every affected post in order to implement it.
If anyone encounters this issue then you can make a more reliable link to the attachment by right clicking on it and selecting "copy link address". Then you can paste the link in your post as an a href or just as a link.