If you upload a file through Firefox, it may send the wrong mimetype for this file.
Delete mimeTypes.rdf, in the user \Application Data\Mozilla\Firefox\Profiles folder.
You may find it in extensions folder also.
Delete the files and restart Firefox and see if it works for you.
http://techblog.procurios.nl/k/news/view/15872/14863/Mimetype-corruption-in-Firefox.html
http://codeigniter.com/forums/viewthread/146338/