Use below code to avoid above problems
<ajax:AsyncFileUpload runat="server" ID="flTripImages"
OnClientUploadStarted="SetTermAgrement" OnUploadedFileError="flTripImages_UploadedError" OnClientUploadComplete="EnableCheckBox"
OnUploadedComplete="flTripImages_UploadedComplete"
oncontextmenu="return false;" />
use oncontextmenu="return false;" to avoid the contextmenu on the upload control, this will take care of above problems
