image button error: System.ArgumentException: Invalid postback or callback argument - asp.net

Question

In the grid view I have uploaded files with image icon. For having image icon in the grid view and getting type of uploaded file in the row (command Argument), I have used image button, it shows the image icon but when I click on the image button, I have error" System.ArgumentException: Invalid postback or callback argument"
If I could use image control instead of image button for me was better, but I do not know how to get extension file type from row.
Also if we could disable link image button in this case, was no problem.
Desin of database for tblfile is like below:
Fileid(int)
Docid(int)
TransId(int)
Content type varchar(75)
Data varbinary(max)
Extention varchar(50)
Cs:
protected void GridViewEfile_DataBound(object sender, EventArgs e)
{
foreach (GridViewRow gvr in GridViewEfile.Rows) //loop each row
{
ImageButton ib = (ImageButton)gvr.FindControl("ib1"); //find imagebutton
string exten = ib.CommandArgument;; //get index of '.'
if (exten == ".pdf")
ib.ImageUrl = #"~/Images/iconpdf.gif"; //set image for imagebutton in each row
else if (exten == ".docx")
ib.ImageUrl = #"~/Images/icondoc.gif";
}
}
protected void ib1_Command(object sender, CommandEventArgs e)
{
_DataContext = new EDMSDataContext();
//you can get your command argument values as follows
string FileId = e.CommandName.ToString();
int _FileId = Convert.ToInt32(FileId);
tblFile Efile = (from ef in _DataContext.tblFiles
where ef.FileId == _FileId
select ef).Single();
if (Efile != null)
{
download(Efile);
}
}
.aspx:
<asp:GridView ID="GridViewEfile" runat="server" AutoGenerateColumns="False" CellPadding="4"
OnDataBound="GridViewEfile_DataBound" ForeColor="Black" DataKeyNames="FileID"
GridLines="None" ShowHeader="False">
<AlternatingRowStyle BackColor="Yellow" />
<Columns>
<asp:TemplateField>
<ItemTemplate>
<asp:ImageButton runat="server" ID="ib1" OnCommand="ib1_Command" CommandName='<%#Eval("FileID")%>'
CommandArgument='<%# Eval("Exten") %>' />
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField>
<ItemTemplate>
<asp:LinkButton ID="LinkButton1" runat="server" OnCommand="LinkButton1_Command" CommandName="Download"
CommandArgument='<%#Eval("FileID")%>'><%#Eval("FileName")%></asp:LinkButton>
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField DataField="ContentType" />
</Columns>
<EditRowStyle BackColor="#7C6F57" />
<FooterStyle BackColor="#1C5E55" Font-Bold="True" ForeColor="White" />
<HeaderStyle BackColor="Red" Font-Bold="True" ForeColor="White" />
<PagerStyle BackColor="#666666" ForeColor="White" HorizontalAlign="Center" />
<RowStyle BackColor="#E3EAEB" />
<SelectedRowStyle BackColor="#C5BBAF" Font-Bold="True" ForeColor="#333333" />
</asp:GridView>