When I attach a file to a Knowledge Doc using the "Attach File ($)" button, the file attaches. I can later see that file in the Administration tab, under Attachments Library, Repositories, Quick Upload. However, when I try to attach files to another Knowledge Doc using the "Attach File From Library" button I cannot see the Quick Upload repository.
This works as designed. Attachment files for one tech doc can not/should not be shared by other tech docs so when you click on button "attach file from library" it will show you the Default repository of Knowledge which is a public repository, not the Quick Upload, which is "private".
If you really want one file be attached to many tech docs, you should upload/add the file to the Default repository.