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!

webhook/webhook.go

@@ -8,10 +8,12 @@
 package webhook
 
 import (
+	"errors"
 	"strings"
 	"time"
 
 	"github.com/mjl-/mox/message"
+	"github.com/mjl-/mox/mlog"
 )
 
 // OutgoingEvent is an activity for an outgoing delivery. Either generated by the
@@ -135,29 +137,43 @@ type NameAddress struct {
 }
 
 type Structure struct {
-	ContentType       string            // Lower case, e.g. text/plain.
-	ContentTypeParams map[string]string // Lower case keys, original case values, e.g. {"charset": "UTF-8"}.
-	ContentID         string            // Can be empty. Otherwise, should be a value wrapped in <>'s. For use in HTML, referenced as URI `cid:...`.
-	DecodedSize       int64             // 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.
-	Parts             []Structure       // Subparts of a multipart message, possibly recursive.
+	ContentType        string            // Lower case, e.g. text/plain.
+	ContentTypeParams  map[string]string // Lower case keys, original case values, e.g. {"charset": "UTF-8"}.
+	ContentID          string            // Can be empty. Otherwise, should be a value wrapped in <>'s. For use in HTML, referenced as URI `cid:...`.
+	ContentDisposition string            // Lower-case value, e.g. "attachment", "inline" or empty when absent. Without the key/value header parameters.
+	Filename           string            // Filename for this part, based on "filename" parameter from Content-Disposition, or "name" from Content-Type after decoding.
+	DecodedSize        int64             // 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.
+	Parts              []Structure       // Subparts of a multipart message, possibly recursive.
 }
 
 // PartStructure returns a Structure for a parsed message part.
-func PartStructure(p *message.Part) Structure {
+func PartStructure(log mlog.Log, p *message.Part) (Structure, error) {
 	parts := make([]Structure, len(p.Parts))
 	for i := range p.Parts {
-		parts[i] = PartStructure(&p.Parts[i])
+		var err error
+		parts[i], err = PartStructure(log, &p.Parts[i])
+		if err != nil && !errors.Is(err, message.ErrParamEncoding) {
+			return Structure{}, err
+		}
+	}
+	disp, filename, err := p.DispositionFilename()
+	if err != nil && errors.Is(err, message.ErrParamEncoding) {
+		log.Debugx("parsing disposition/filename", err)
+	} else if err != nil {
+		return Structure{}, err
 	}
 	s := Structure{
-		ContentType:       strings.ToLower(p.MediaType + "/" + p.MediaSubType),
-		ContentTypeParams: p.ContentTypeParams,
-		ContentID:         p.ContentID,
-		DecodedSize:       p.DecodedSize,
-		Parts:             parts,
+		ContentType:        strings.ToLower(p.MediaType + "/" + p.MediaSubType),
+		ContentTypeParams:  p.ContentTypeParams,
+		ContentID:          p.ContentID,
+		ContentDisposition: strings.ToLower(disp),
+		Filename:           filename,
+		DecodedSize:        p.DecodedSize,
+		Parts:              parts,
 	}
 	// Replace nil map with empty map, for easier to use JSON.
 	if s.ContentTypeParams == nil {
 		s.ContentTypeParams = map[string]string{}
 	}
-	return s
+	return s, nil
 }