This is probably the result of confusion about the correct character encoding for the file that causes the XMI reader to not recognize the XML header. Either that or the file is just corrupt in some way.
What character encoding is used for the file? Does the file header look well formed? Will it open in another XML reader like a web browser?
The best way to get help if you're still having problems is to fill out a bug report at and attach the file.
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