Support creating Document attachments

In particular, this will support audio attachments.

Ref T872

activitystreams/attachment.go

@@ -14,20 +14,34 @@ type Attachment struct {
 
 type AttachmentType string
 
-const AttachImage AttachmentType = "Image"
+const (
+	AttachImage    AttachmentType = "Image"
+	AttachDocument AttachmentType = "Document"
+)
 
 // NewImageAttachment creates a new Attachment from the given URL, setting the
 // correct type and automatically detecting the MediaType based on the file
 // extension.
 func NewImageAttachment(url string) Attachment {
-	var imgType string
+	return newAttachment(url, AttachImage)
+}
+
+// NewDocumentAttachment creates a new Attachment from the given URL, setting the
+// correct type and automatically detecting the MediaType based on the file
+// extension.
+func NewDocumentAttachment(url string) Attachment {
+	return newAttachment(url, AttachDocument)
+}
+
+func newAttachment(url string, attachType AttachmentType) Attachment {
+	var fileType string
 	extIdx := strings.LastIndexByte(url, '.')
 	if extIdx > -1 {
-		imgType = mime.TypeByExtension(url[extIdx:])
+		fileType = mime.TypeByExtension(url[extIdx:])
 	}
 	return Attachment{
-		Type:      AttachImage,
+		Type:      attachType,
 		URL:       url,
-		MediaType: imgType,
+		MediaType: fileType,
 	}
 }