Wenn die Datei nicht im Dateieingangstyp PHP ausgewählt ist

We Could Use

For Single file:

if ($_FILES['logo']['name'] == "") {
    // No file was selected for upload, your (re)action goes here
}
For Multiple files:

if ($_FILES['logo']['tmp_name'][0] == "") {
    // No files were selected for upload, your (re)action goes here
}
Ankur