Add PDF support to save_all_attachments.py

Line 130 results in .xxx if not .txt if I read it right,
I changed it to 

```python
if part.get_content_type() == 'text/plain':
    ext = '.txt'
elif part.get_content_type() == 'application/pdf':
    ext = '.pdf'
else:
    ext = '.xxx'
```
to ensure to get .pdf extension if it is a PDF, I know this is not originally from you, but I just found it here. Thank you.