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Won't do - possibly never.
The issue here is in storage of attachments, their availability and the usage.

• If tasks are created on schedule in intervals, without editing it before creation of task... all would have same attachment - no point in keeping that attachment with template if its the same for all created issues - more effective is to keep it one place always available, even for changes in the attachment, then if you change it you don't need to change it in SI, because the link would be already correct eg. to some download section
• If attachment needs to be different in each task, it is still better to update created issue than update template before every execution - there could be just a note (or subtask) in issue to add appropriate attachment
• If it's a task for 1 issue which will be created in future only 1 time - again there is a storage problem, and the task should probably be created in Jira 'now', but with start date in future - because if you already have attachment for that task, looks like there is only need to set date when to start working on it.