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

What common mistake do developers make with Upload security, why does the mistake cause problems, and what is the safer approach? One recurring problem with upload security is applying it without understanding the boundary it protects. Trusting the original client filename or extension as proof that an uploaded file is safe. In addition, File validation, storage isolation and least-privilege access are important because uploaded content can be attacker-controlled. 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.