yeah D, i had the same problem, in my case i have 2.5.6 sigpro version and the bug is in 2868 line from /administrator/components/com_k2/lib/class.upload.php

$this->file_src_mime = mime_content_type($this->file_src_pathname);
x
$this->file_src_mime = mime_content_type($this->file_src_name);

thanks!!