Thank you for contacting Telerik support.
Straight to the points:
RadAsyncUpload can upload multiple files when either Adobe Flash Player (v.10 or newer) or Silverlight for the file selection dialog to allow selecting multiple files. If neither Flash nor Silverlight is available, RadAsyncUpload will gracefully degrade to single file selection as on this on-line demo.
For your major question about the Long Server Process -so far we are not aware of any issues with RadAsyncUpload concerning such problems so I will recommend you to test it yourself in similar scenario or elaborate a little bit how we could reproduce the same behavior. I am attaching a sample test web page for this purpose.
It can work in UpdatePanel or when RadAjaxManager is used as it is shown in this demo.
As for the questions about the "can send an update back to the client-side so it knows it got a response" would you plea elaborate a little bit what should this issue do because it is no quite clear?
Hope this information will be helpful. Please let us know if you have further questions.
the Telerik team
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