### Summary
Closes#2489, which reported an inability to process `.p7s` files. PR
implements two changes:
- If the user selected content type for the email is not available and
there is another valid content type available, fall back to the other
valid content type.
- For signed message, extract the signature and add it to the metadata
### Testing
```python
from unstructured.partition.auto import partition
filename = "example-docs/eml/signed-doc.p7s"
elements = partition(filename=filename) # should get a message about fall back logic
print(elements[0]) # "This is a test"
elements[0].metadata.to_dict() # Will see the signature
```
