Add sections for encoding, file extension, and mime-type

Author: achow101

bip-0174.mediawiki

@@ -329,6 +329,13 @@ The Transaction Extractor does not need to know how to interpret scripts in orde
 
 A single entity is likely to be both a Transaction Extractor and an Input Finalizer.
 
+==Encoding==
+
+A PSBT can be represented in two ways: in binary (as a file) or as a Base64 string using the encoding described in [https://tools.ietf.org/html/rfc4648#section-4 RFC4648].
+
+Binary PSBT files should use the <tt>.psbt</tt> file extension.
+A MIME type name will be added to this document once one has been registered.
+
 ==Extensibility==
 
 The Partially Signed Transaction format can be extended in the future by adding