Please check following
1. Allow attachment in Front End PM PRO Settings
2. File mime type is present and it is in allowed type
3. File size is less then from Front End PM PRO Settings
You can also remove some check from this function and try to find out which check failed. eg remove
if( !$mime || !in_array( $mime, get_allowed_mime_types() ) ) return false;
then try. Also remove other check and try.