### 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
```
The sample docs directory contains the following files:
example-10k.html - A 10-K SEC filing in HTML format
layout-parser-paper.pdf - A PDF copy of the layout parser paper
factbook.xml/factbook.xsl - Example XML/XLS files that you can use to test stylesheets
These documents can be used to test out the parsers in the library. In addition, here are
instructions for pulling in some sample docs that are too big to store in the repo.
XBRL 10-K
You can get an example 10-K in inline XBRL format using the following curl. Note, you need
to have the user agent set in the header or the SEC site will reject your request.
curl -O \
-A '${organization} ${email}'
https://www.sec.gov/Archives/edgar/data/311094/000117184321001344/0001171843-21-001344.txt
You can parse this document using the HTML parser.
ccf0477080
