FileUpload Control Inside Gridview (ajax Updatepanel)

I've got a fileupload control inside of a gridview so that each row (item from the database) can have an attachment associated with it. I've got a button next to the fileupload control (besides the default browse... button to select the file) to actually take the file and upload it to a file server. This works great if I use this button "Upload". However, underneath my gridview I have another imagebutton that is outside of the gridview, its just sitting on the form / page. The idea of this "Update" button is when end users make changes to the grid I can simply loop through the grid and update the values they have entered to the database. This also works great.

Now here is my issue assume a user has 3 rows in the grid view. On 2 of these rows he decides to add an attachment but he never clicks the "Upload" button he only uses the standard browse button to select the file. Now instead of clicking the "Upload" button that is on the grid view he ends up clicking the "Update" button outside of the grid thinking that it will save his attachments. So I decided to write code to grab the fileupload control from the grid using .FindControls("NameOfFileUploadControl") in my Update button loop where I loop through all the rows. Then I check if myFileUploadControl.HasFile property to see if there exists a file and if there does upload the file then continue with the code.

The issue is when I click this "Update" button .HasFile always returns false even though I can see the text string of the path in the fileuploadcontrol textbox (the standard one next to the browse button). Is it because this goes back to the page load event and checks if it is a post back? What causes this and how can I fix it? Im adding some code because im still confused with this

Protected Sub SaveGrid()
For Each Row As GridViewRow In Me.gvLineItems.Rows
Dim f As FileUpload = CType(Row.FindControl("fuAttachment"), FileUpload)
'todo: this part is not working
If f.FileName.Length > 0 Then 'returns 0 always
'this returns 0.
end if
if f.HasFile() then
'this also returns false
end if
Next
End Sub

This method gets called on a click of a button. The fileupload control is in an of a gridview that is wrapped inside of an UpdatePanel:

<asp:FileUpload Width="90px" Font-Size="xx-small" ID="fuAttachment" runat="server" />