That's a great question. The original meaning of dosiero is like a file, which is often kept in a folder -- right, so we may have a file about a particular client where we keep several papers together about that one client. So, a dosierujo would be a container for files - possibly a big box of files, or perhaps a filing cabinet. (PIV says "cardboard box" but I don't think it's that limited.)
Commonly today, however, we think of a file as something on a computer, and a dosierujo as a place to put these files. In English we put computer files in a "folder".
So yeah, it's funny to me that when we're talking physical files, a folder is the a place for a single file, but when we're talking about computer files a folder is like the whole box.