You miss few steps. The name of file is not the same with name of category. In same file you could have more category.
After you load the furniture in furniture library editor, select all and then click on modify selected furniture (the 8 icon from left); in the new window edit the category name. You must see it in the category column (the 4 col.). Now you can save Your library under whatever name you want. When you import it in sh3d will see the category name not the name of the file.
dorin