[kh99]

I don't know all the details, but basically each file has a row in filedata. If you are storing files in the database, then the actual file contents go in there, otherwise they go in the filesystem, and I believe the filename is related to the fileid. Then there's a row in attachment for each place it's attached (the same file can be attached more than one place). contenttype tells what it's attached to (a post or somewhere else, I'm no sure what all the possibilities are), and contentid has the postid or whateevr other id is appropriate. Also, in the post table I believe the attach column is the count of attachments to each post.

I don't know what attachmentcategoryuser is for.

Sorry I don't have more details off the top of my head, maybe someone else will have more to say.