RadUpload does not lock the saved with the SaveAs method or the accessed with the InputStream property files. The files are automatically closed by the RadUpload control. You can take a look at this help article
for more information about manipulating the uploaded files and some sample code snippets.
Probably there is some other process in your case which locks the uploaded files after they are saved. From the provided code snippet I am not able to find which is the process which causes the problem. You should research and find what locks the uploaded files in order to prevent this happening.
If the suggested above cannot help you solve the problem, please, send us a simple running application which reproduces the issue so that we can take a closer look at your scenario.
You will need to open a support ticket in order to have the right to attach files. Please, download a trial version of the RadControls for ASP.NET AJAX suite and you will have the right to open a support ticket. For your convenience I have attached a screenshot with instructions how to open a support ticket.
the Telerik team