Does the filename contain any spaces? Any files I attached that contained spaces in the name were stripped of all characters after the space (including the extension).
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In any case, if that turns out to be your problem, a quick fix is just to replace all space characters with an underscore.
To do this, open ~/include/class.file.php, find:
$filename=basename($this->getName());
and replace it with
$filename=basename($this->getName());
$filename = str_replace(" ", "_", $filename);