Update content-type handling for FreeBSD in msgwriter test

Adjusted the expected content-type header in msgwriter tests to account for FreeBSD by setting it to 'application/octet-stream' instead of 'text/plain'. This ensures compatibility and correct behavior across different operating systems.

msgwriter_test.go

@@ -346,8 +346,15 @@ func TestMsgWriter_addFiles(t *testing.T) {
 			if msgwriter.err != nil {
 				t.Errorf("msgWriter failed to write: %s", msgwriter.err)
 			}
-			ctExpect := fmt.Sprintf(`Content-Type: text/plain; charset=utf-8; name="%s"`, tt.expect)
+
+			var ctExpect string
 			cdExpect := fmt.Sprintf(`Content-Disposition: attachment; filename="%s"`, tt.expect)
+			switch runtime.GOOS {
+			case "freebsd":
+				ctExpect = fmt.Sprintf(`Content-Type: application/octet-stream; charset=utf-8; name="%s"`, tt.expect)
+			default:
+				ctExpect = fmt.Sprintf(`Content-Type: text/plain; charset=utf-8; name="%s"`, tt.expect)
+			}
 			if !strings.Contains(buffer.String(), ctExpect) {
 				t.Errorf("expected content-type: %q, got: %q", ctExpect, buffer.String())
 			}