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The Flash uploader has been replaced with a JS one. You only need to make sure you haven't missed the /js folder in the upgrade, and add this to your layout file:

    <reference name="head">
            <action method="addJs"><file>lib/uploader/flow.min.js</file></action>
            <action method="addJs"><file>lib/uploader/fusty-flow.js</file></action>
            <action method="addJs"><file>lib/uploader/fusty-flow-factory.js</file></action>
            <action method="addJs"><file>mage/adminhtml/uploader/instance.js</file></action>
</reference>