Variable $_FILES don’t returns value when posting a form from PHP

I am trying to send file info from a PHP form, but it doesn’t appears.

I have a file with this form:

<form action="procesos/procesar_entradas.php" method="post" class="form-horizontal" id="entradasForm" enctype="multipart/form-data">
[...]
<input type="file" accept=".png, .jpeg, .jpg, .gif" name="foto_muestra" id="foto_muestra" class="form-control" />
<input type="file" accept=".png, .jpeg, .jpg, .gif" name="foto_monton" id="foto_monton" class="form-control" />
[...]
<button type="submit" class="btn btn-default">Registrar entrada</button>
</form>

I am uploading two images. When I click submit, I go the action file marked. It reaches here correctly.
But when I try to know about $_FILES, this is empty, but I am using these images. Why does returns it empty?

How can I indicate to the form that I want to get these files this way?:

$_FILES['foto_muestra']
$_FILES['foto_monton']

Thanks in advance, cause it comes empty and I don’t know what to do.

To get $_FILES not empty, with image data sent from form. Multipart is ON.