What common mistake do developers make with Attachments, why does the mistake cause problems, and what is the safer approach?

What common mistake do developers make with Attachments, why does the mistake cause problems, and what is the safer approach? One recurring problem with attachments is applying it without understanding the boundary it protects. Hard-coding SMTP passwords or API keys inside a Mailable class instead of using environment-backed configuration. In addition, Mailables can include supported file attachments when the application needs invoices or documents. Safer engineering approach: keep the responsibility in the correct layer, use the framework's supported API, validate and authorize inputs, avoid hidden side effects, and verify the behavior with tests.