I have just discovered (after about six months) why when using the
Flash uploader for images on Firefox on Ubuntu, it usually can't see the file I want to upload.
It's because the file extension has to be lower case, i.e. *.jpg not *.JPG.
I could not find this documented anywhere. The problem started when I moved from Macintosh to Ubuntu. (Probably because my new graphics editor preserves the case of the file extension I have been editing.)
I kept seeing these messages saying "You are using the Flash uploader. Problems? Try the Browser uploader instead.", so I just assumed the Flash one was very buggy.