could be a php error occuring.
to trouble file uploads, follow these steps:
1) Add the following code a t line 1:
2) Make sure there are no other server behaviors on the page that may redirect to another page.
3) On the bindings panel, expand the WA Upoad Files Global Status section and bind the Debug info to the page.
Post back the debug info after trying an upload along with any errors and a copy of the page.