mime type check

It'd be nice to be able to validate certain file types, e.g. shared objects, dynamically linked executables, static executables, and executables for different architectures.
It's currently possible to do with `file`, but it's a big dependency to pull in and it's normally not present in minimal images.

Example:

```YAML
fileContentTests:
- name: "bash is an executable binary"
path: "/bin/bash"
mimeType: 'application/x-executable'
isExecutable: true

elfBinaryTests:
- name: "bash is dynamically linked arm64 binary"
path: "/bin/bash"
isDynamicallyLinked: true
architecture: "amd64"
```