I wouldn't say Flow Designer is a better option in this case, especially if you're more familiar with and already running a Workflow on the catalog item. If you must copy attachments, the easiest/best place to do this is in a Business Rule running on the sc_task table after Insert. All you need is this code in the Advanced Script.
(function executeRule(current, previous /*null when async*/) {
GlideSysAttachment.copy('sc_req_item', current.request_item, 'sc_task', current.sys_id);
})(current, previous);
The reasons for not copying attachments are that it bloats the database with the same attachment record multiple times, and if the attachment is updated on the RITM or SCTASK, the other is not changed. A better option is to create a Defined Related List on the SCTASK form to show the attachments for the parent sc_req_item.
https://docs.servicenow.com/bundle/kingston-platform-user-interface/page/administer/form-administration/task/t_CreateDefinedRelatedLists.html
