add Content-Disposition and Filename to the payload of incoming webhooks

for each message part. The ContentDisposition value is the base value without
header key/value parameters. the Filename field is the likely filename of the
part. the different email clients encode filenames differently. there is a
standard mime mechanism from rfc 2231. and there is the q/b-word-encoding from
rfc 2047. instead of letting users of the webhook api deal with those
differences, we provide just the parsed filename.

for issue #258 by morki, thanks for reporting!

webaccount/api.json

@@ -1495,6 +1495,20 @@
 						"string"
 					]
 				},
+				{
+					"Name": "ContentDisposition",
+					"Docs": "Lower-case value, e.g. \"attachment\", \"inline\" or empty when absent. Without the key/value header parameters.",
+					"Typewords": [
+						"string"
+					]
+				},
+				{
+					"Name": "Filename",
+					"Docs": "Filename for this part, based on \"filename\" parameter from Content-Disposition, or \"name\" from Content-Type after decoding.",
+					"Typewords": [
+						"string"
+					]
+				},
 				{
 					"Name": "DecodedSize",
 					"Docs": "Size of content after decoding content-transfer-encoding. For text and HTML parts, this can be larger than the data returned since this size includes \\r\\n line endings.",