if you want to upload file and save as string on server then you should use upload file and use stream as byte array and convert it to base64 string
you can use following code
<asp:fileupload id="flupUpload" runat="Server" />
<asp:button id="Button1" runat="server" onclick="Button1_Click" text="Button" />
protected void Button1_Click(object sender, EventArgs e)
{
int iLen = flupUpload.PostedFile.ContentLength;
byte[] btArr = new byte[iLen];
flupUpload.PostedFile.InputStream.Read(btArr, 0, iLen);
Response.Write(Convert.ToBase64String(btArr));
}
mark as solution if you problem solved