Added email-related notes to the knowledge base

knowledge base/extract attachments from multipart emails.md

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+# Extract attachments from multipart emails
+
+When saved as plain text, emails may be saved as S/MIME files with attachments.  
+In such cases the text file itself contains the multipart message body of the email, so the attachments are provided as base64 streams:
+
+```text
+ 1 --------------060903090608060502040600
+ 2 Content-Type: application/x-gzip;
+ 3  name="myawesomefile.tar.gz"
+ 4 Content-Transfer-Encoding: base64
+ 5 Content-Disposition: attachment;
+ 6  filename="myawesomefile.tar.gz"
+ 7
+ 8 qrsIAstukk4AA+17Wa+jSpZuPfMrrK6Hvi1qJ4OxsfuodQU2YLDBUJDGrvqBeR7MaPj1N7D3
+ 9 OEmSxO8Wq7+3Y48dTWvXi8XvvKj8od6vPf9vKjWIv1v7nt3G/d8rEX5D/FdrDIxj2IrUPeE/
+10 j5Dv4g9+fPnTRcX006T++LdYYw7w+i...
+```
+
+You can use `munpack` to easily extract attachments out of such text files and write them into a proper named files.
+
+```shell
+$ munpack -f plaintext.eml
+myawesomefile.tar.gz (application/x-gzip)
+```
+
+## Sources
+
+- [Extract attachments from multipart messages]
+
+[extract attachments from multipart messages]: https://liquidat.wordpress.com/2013/08/07/short-tip-extract-attachments-from-multipart-messages/