The webhook in JIRA does not contain any information about the attachment if the issue was created using email. However, it does contain the attachment information if the issue was created using UI.

Steps to reproduce.

• Set up a webhook in JIRA for 'Issue Created' event.
• Now create an issue using the JIRA UI in a way that the webhook event will be triggered.
• Notice that in the JSON, there is attachment information present.
• Now create another issue with the same attachment but using through email
• Notice that in the JSON, there is no attachment information present
• The issue created has the attachment present.

(See attachments)

On a side note :
If issue is moved from one issuetype to another issue type that is sending information via webhook then also the attachment are not sent. But if those attachments are added after movement then they are sent via webhook.