This happens if the allowed file types is missing. Go to your dashboard and click systems and settings. Now click on allowed file types in the permissions and access area and check to see that you have some file types there, if you dont than copy and paste this in,

flv, jpg, gif, jpeg, ico, docx, xla, png, psd, swf, doc, txt, xls, xlsx, csv, pdf, tiff, rtf, m4a, mov, wmv, mpeg, mpg, wav, avi, m4v, mp4, mp3, qt, ppt, pptx, kml, xml


A screenshot is attached

After you should see an upload button in your file manager.