read name of selected file

i use input(type="file") tag in one of my forms like this:

<span class="btn fileinput-button btn-success">
<span>select files</span>
<input type="file" id="fileupload" name="files[]" multiple>
</span>

i want to read the selected file name in a another java script function. i want to read the selected file name here in the java script function to use as a parameter:

var req="ajax/updatereguser.php?uname="+txtuname+"&avatar="+txtavatar+"& bdate="+txtbdate+"&sex="+txtsex+"&lang="+txtlang+"&country="+txtcountry+"&phone="+txtphone;
xmlhttp.open("GET",req,true);
xmlhttp.send();

or read that and use as a parameter in the following code:

<button type="button" class="btn btn-success" name="btncontinue" id="btncontinue" onClick="updateregusers(txtparameter)">Continue</button>

how can i do this? thanks