Notice: Undefined index: ($_FILES) - php

Question

There are multiple questions[duplicates] about this same error but I can't find a solution in any of them. I`m trying to save an image in my local directory but 'Undefined index' keeps showing up. I´m a noob in php and stack overflow. I would really appreciate any of your help.
HTML:
<form method="post" action="process.php" enctype="multipart/form-data">
<fieldset>
<div class="col-xs-12 col-sm-push-5 image-div form-control">
<div class="input-margin">
<input type="file" name="imagenIngreso" class="filestyle" onchange="readURL(this);" data-buttonText="Subir Imagen" data-buttonName="btn-primary" data-size="nr" data-buttonBefore="false">
</div>
<div>
<img class="image-preview" id="blah" src=""/>
</div>
</div>
<div class="col-xs-12 col-sm-12 submit-button">
<button type="submit" class="btn btn-primary btn-block" name="submit" value="ingreso">Ingresar</button>
</div>
</fieldset>
</form>
PHP:
// Previous Validations
else if($_POST['submit'] == 'ingreso') {
$fileName = $_FILES['imagenIngreso']['name']; // Line throwing error
$uploadDirectory = "images/uploadedImages/Ingresos/";
if($uploadedFileName!='') {
$targetPath=time().$uploadedFileName;
if(move_uploaded_file($_FILES['imagenIngreso']['tmp_name'], $uploadDirectory.$targetPath)) {
$queryIngresos = "INSERT INTO ingresos (fechaIngreso, responsable, proyecto, items, imagePath) VALUES (STR_TO_DATE('$fechaEgreso','%d/%m/%Y'), '$responsable', '$proyectoIngreso', '$itemsArray', '$targetPath')";
if(!mysqli_query($con, $queryEgresos)) {
echo "Error while uploading file";
die('Error :'.mysqli_error($con));
} else {
echo 'Upload done';
exit();
}
}
}

Answer 1

You can also find what error are you getting using the Error Codes available.
Put this and make a try.
<?php
if($_FILES['userfile']['error']) {
// handle the error
} else {
// process
}
?>