Update upload.php

It seems logical to add this line, since such verification exists for $storeFolder.

Also replaced $_POST['path_thumb'] with descriptive name, and removed that +$thumb_pos and +$path_pos because at this lines we are already sure they are equal to 0.

Besides that, I'm not sure if this code does any useful thing:
        else{
	    move_uploaded_file($tempFile,$targetFile);
	    chmod($targetFile, 0755);
	}

because we are already moved uploaded file at the beginning (lines 60-61)