Validate if the field of a form is empty

[te0t3l <te0t3l@xxxxxxxxx>]

Hi, I need to validate a field that work with Multifile plugin of Jquery,
I want to check if the field is empty with php.

[code]
<input name="archivo[]" class="multi" type="file" accept="gif|jpg|png"/>
<input name="button" type="submit" value="Submit">
[code]

I've tried different ways but it does not work:
for example:
$field = $_FILES["archivo"]["name"];
$field = $_FILES["archivo[]"]["name"];

i dont know how to validate it, any help would be appreciated.

Thanks,