The RadUploadHandler is a plain .NET ClassLibrary and it can be build for .NET 3.5 without any modification (just the project build action). However we does not ship our binaries build for .NET 3.5 from a long time and in order to get the Handler for .NET 3.5 you will need to build it locally using our source code.
You can download the source code from you account. Open the download page, find the UI for Silverlight and press the "Browse all product files" the source code is located in the bottom of the page.
After that take a look at the build instructions located inside the SourceFolded->Build->BuildInstructions, following these instruction you should
n't have any problems rebuilding our source. Note that the RadUploadHandler project is located in Telerik_UI_for_Silverlight_Source\Controls\Input\RadUploadHandler.
I hope this information helps.
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