To upload a file, use the "Upload file" link at the bottom of any page (you must be logged in first).
Currently the following extensions are accepted:
png, gif, jpg, jpeg, doc, xls, mpp, pdf, ppt, tiff, bmp, docx, xlsx, pptx, ps.
Inserting a file or a picture in an article
Uploaded files and pictures can be inserted into an article: see Inserting Files and Pictures.