Dropzone and asp elements

i'm quite new developing for asp.net and found a problem i can't figure it out. My .aspx look like the follow:

<form action="#" class="" runat="server" >
    <asp:DropDownList ID="ddlCombo" runat="server">
    </asp:DropDownList>
</form>

<div class="clsbox-1" runat="server">
    <div class="dropzone clsbox" id="mydropzone">
    </div>
</div>

As you can see i have a dropdownlist that i will populate on page_load and a dropzone inside a div (i need to be this way).

<script>
Dropzone.autoDiscover = false;
$("#mydropzone").dropzone({
    url: "WebForm1.aspx?param=1",
    addRemoveLinks: true,
    maxFilesize: 0.5,
    dictDefaultMessage: 'drag and drop',
    dictResponseError: 'Error uploading file!'
});
<script>

this is the script that make div like a dropzone, i'm not sure if the url is right though. The problems comes when in the page_load part, since i need to populate the dropdownlist on page load, when i try to upload a file via dropzone it will be sent as postback and enter in the 'if' and populate again the combobox, reseting the value that was selected.

protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack) {
       loadCombo(); //populate ddlCombo
    }
    string aux = ddlCombo.SelectedValue.ToString();//this will have the first item

   //processing file...
}

Please let me know if there is any fancy way to do this because i can't figure it out!

EDIT: I don't understand why if i send within a form it work as intended:

<form action="#" class="dropzone" runat="server" >
    <asp:DropDownList ID="ddlCombo" runat="server">
    </asp:DropDownList>
    <div class="clsbox-1 " id="mydropzone" runat="server">
        <div class="clsbox" >
        </div>
    </div>
</form>

When i upload a file here it doesn't send as post back. Thx you!

Your dropzone url parameter should be to a different page/httphandler/controller you have created to accept the upload and save it to a file (or whatever you want to do with it). You could technically use the page you're currently on, but it'd be a little trickier in your situation as you'd need to detect what caused the postback and handle appropriately (ie don't reset the dropdown).

I would recommend using an HttpHandler instead of a normal page for receiving the files. Here's an example of one.