You can upload files by clicking the Add File button or by simply dragging & dropping a file from your desktop to the folder.
The files are uploaded directly to the current folder you are in.
Utilize shift+click or command+click to select multiple files to upload at one time.
Drag-and-drop files directly from your desktop to upload files.
To upload a file to a specific folder, drag the file to the folder.
When the folder is highlighted in blue drop the file. You are redirected to the folder and the file is uploaded there.