Add test on MailFile parsing failure

fs/maildir_test.go

@@ -2,6 +2,7 @@ package fs
 
 import (
 	"fmt"
+	"strconv"
 	"testing"
 	"time"
 
@@ -75,3 +76,22 @@ func TestNewMailFile(t *testing.T) {
 		t.Errorf("MailFile mismatch: %s", cmp.Diff(expect, mf, cmp.AllowUnexported(MailFile{})))
 	}
 }
+
+func TestNewMailFileErrs(t *testing.T) {
+	var err error
+
+	// The name has the wrong shape
+	_, err = NewMailFile("pippo")
+	if err != ErrMalformedName {
+		t.Error("Function should have errored")
+	}
+
+	// The shape is correct, but one of the integer field is instead a string
+	_, err = NewMailFile("aaaa_123.456.myplace,U=1312,FMD5=f2cf513ad46d4d9b9684103e468803a0:2,")
+	switch err.(type) {
+	case *strconv.NumError:
+		// Happy path
+	default:
+		t.Error("Unexpected error")
+	}
+}