There can be many reason why an file upload fails, to get more information about the file upload you will need to add the debug info to your page. This should tell you more about the upload and the problems occurring. You can get to the debug info from the bindings tab under the all uploads entry.
One common problem with uploads is the size of the file. This is covered in the FAQ area for Universal Email along with some other common issues. Please take a look at this for more information about uploading.