If you are directly in Singlecase on a specific case file, you can use the "Cloud" button at the top.

You will then be presented with a standard selection window where you can find the files you want to select.

Select a file (or multiple files) by selecting it with the mouse or by holding down the "ctrl" ("cmd") key. Then type "Open" and the relevant files will be uploaded under Project/Case in Documents. The files are not indexed and can be found at the end of the folder structure.