[HTML]

var value = document.getElementById('prod_image[]').value;

[/HTML]

That should get you the value of the input of id "prod_image[]".

The only problem that I can think of is if you have more than one id of "prod_image[]".

Example:

[HTML]<input name="prod_image[]" type="file" class="Textfield" id="prod_image[]"/>
<input name="prod_image[]" type="file" class="Textfield" id="prod_image[]"/>
<input name="prod_image[]" type="file" class="Textfield" id="prod_image[]"/>[/HTML]

Then document.getElementById('prod_image[]') will only return the first input element.

If you have multiple inputs like above you can iterate through them using the DOM method, getElementsByTagName().

Eg:

[HTML]var inputs = document.getElementsByTagName('input');

for (var i = 0; i < inputs.length; i++) {

if (inputs[i].type == 'file') { // check if this is a file
// do your validation here...

}

}[/HTML]

The problem with method is that you will get all input elements in the document via document.getElementsByTagName('input'); while all you want are the input elements in the form you are validating.
To get elements only in the form you will use:

[HTML]document.forms['form_name'].getElementsByTagName('input');[/HTML]

where 'form_name' is the name of your form.