Fix for content type probing for static files.

[kothar]

Issue #254

Description of changes:
Prefix non-absolute paths with "/tmp" when probing MIME type. This is a working fix to #254 provided as an example, but may need some additional randomisation adding to the path to avoid any potential collision with actually existing files in /tmp. Also not yet compared to behaviour of frameworks other than Spring Boot.

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[sapessi]

Perhaps instead of going to /tmp we should point to the task root (/var/task). I'm planning to add a getTaskRoot method to the ContainerConfig object that relies on the Lambda environment variable when available.

[kothar]

@sapessi, assuming that the path requested ('favicon.ico' for example) won't be a literal mapping onto a file in /var/task (more likely '/var/task/static/favicon.ico' or embedded inside a jar file), do you agree that it should be assumed we aren't trying to map to a real file, and ensure that the path is randomised to avoid leaking any information about real files in the function package?

Any path under /var/task or /tmp would pass the security check, and I agree /var/task is probably preferable.

[sapessi]

Agree 100% on not leaking information @kothar. That's why I'd rather avoid /tmp. The temp directory is writable inside Lambda and may be used to store some state. I'll do some digging to figure out if there's a way to get the absolute path out of Spring.

[sapessi]

Closing this PR since the fix was implemented based on the discussion on issue #254