diff --git a/activitystreams/attachment.go b/activitystreams/attachment.go
index b2b47e9..c2bfc2e 100644
--- a/activitystreams/attachment.go
+++ b/activitystreams/attachment.go
@@ -1,12 +1,33 @@
 package activitystreams
 
+import (
+	"mime"
+	"strings"
+)
+
 type Attachment struct {
 	Type      AttachmentType `json:"type"`
 	URL       string         `json:"url"`
 	MediaType string         `json:"mediaType"`
 	Name      string         `json:"name"`
 }
 
 type AttachmentType string
 
 const AttachImage AttachmentType = "Image"
+
+// 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
+	extIdx := strings.LastIndexByte(url, '.')
+	if extIdx > -1 {
+		imgType = mime.TypeByExtension(url[extIdx:])
+	}
+	return &Attachment{
+		Type:      AttachImage,
+		URL:       url,
+		MediaType: imgType,
+	}
+}
diff --git a/activitystreams/attachment_test.go b/activitystreams/attachment_test.go
new file mode 100644
index 0000000..c96d1cd
--- /dev/null
+++ b/activitystreams/attachment_test.go
@@ -0,0 +1,40 @@
+package activitystreams
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestNewImageAttachment(t *testing.T) {
+	type args struct {
+		url string
+	}
+	tests := []struct {
+		name string
+		args args
+		want *Attachment
+	}{
+		{name: "good svg", args: args{"https://writefreely.org/img/writefreely.svg"}, want: &Attachment{
+			Type:      "Image",
+			URL:       "https://writefreely.org/img/writefreely.svg",
+			MediaType: "image/svg+xml",
+		}},
+		{name: "good png", args: args{"https://i.snap.as/12345678.png"}, want: &Attachment{
+			Type:      "Image",
+			URL:       "https://i.snap.as/12345678.png",
+			MediaType: "image/png",
+		}},
+		{name: "no extension", args: args{"https://i.snap.as/12345678"}, want: &Attachment{
+			Type:      "Image",
+			URL:       "https://i.snap.as/12345678",
+			MediaType: "",
+		}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := NewImageAttachment(tt.args.url); !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("NewImageAttachment() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}