Have searched quite for some hours but I got finally a great solution.
You may find original post Here.
Here a short summary:
1.Edit file timthumb.php location: wp-content/themes/arras-theme/library
2.Go to line 370 -(text say: Use PECL fileinfo ...)
3.uncommnet following code lines:
// if (function_exists('finfo_open')) {
// $finfo = finfo_open(FILEINFO_MIME);
// $mime_type = finfo_file($finfo, $file);
// finfo_close($finfo);
// }
add following:
$file_cmd = "file -ib \"$file\"";
exec($file_cmd, $exec_output);
$mime_type = $exec_output[0];
4. Save the file.
That worked for me. Hope it does for you too.