attachments are held in root/media/k2/attachments
when you upload a file to the first item its name is the same as you chose from your local machine
when you upload the same file to another item its name is the same_1.extension
so, if you use the upload to put the files there it will not work.
if you look in the table with phpmyadmin you will see that the table jos_k2_attachments contains a file name and also an itemid that it links to, so you can attach multiple files to a single item.
if you dont have many attachments like this you might be able to attach a dummy file, and then edit the table and change the file name and it may work.
it wasnt designed this way, and the core would need to be hacked to change this.