Use sanitize_file_name( $filename )
. This function doesn’t just catch white space, it removes other special characters too:
$special_chars = array(
"?", "[", "]", "https://wordpress.stackexchange.com/", "\\", "=", "<", ">", ":", ";", ",",
"'", "\"", "&", "$", "#", "*", "(", ")", "|", "~", "`",
"!", "{", "}", chr(0)
);
Find the following line in the plugin’s code:
$id = media_handle_sideload($file_array, $post_id, $desc);
Immediately above this line add:
$file_array['tmp_name'] = sanitize_file_name( $file_array['tmp_name'] );
Caveat: I haven’t tested it.
Make sure to rename the plugin and the plugin’s file name. Otherwise you get upgrade notices. And contact the author, ask him to implement these changes.