After upgrading to Moodle 2.4 the Metalink block no longer displays the choose a file button. Instead there is just a spinning circle with a mouse over tool tip of "Loading...".

The Upload button is still there but just makes a small popup which shows:
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Upload Metacourse links

[[noform]]
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I have tested it in IE9, Firefox 17 and Chrome 24.0.1312.56 all show the same spinning circle with no selection option.