First under which platform are u workin ???
win ou linux/unix
You must ensure that your form in html has a type properly seted.
than the second reason could be that the files that you are tryin to upload is freater than
MAX_FILE_SIZE
just grow this value under your form
<input hidden .... value="10000000".....>
the last reason could be the max size of file that can be uploaded, you'll find this value in conf/httpd.conf
i hope that this will help u
good luck