File uploads do not work when doing an async postback. To my knowledge there's no way to support this scenario due to the browser's security restrictions.
Reason - understand how async postbacks are different than a regular postback.
When the browser performs a Regular Postback it has the benefit of being actual code with no security restrictions running on the client computer. When you submit a form it creates a web request with all the form fields. When it encounters an <input type="file"> it examines the file the user chose, reads it from disk, and adds it to the web request.
· Have a dedicated "Upload" button that does a regular postback instead of an async postback. You can achieve this using several techniques:
o Have the button be outside all UpdatePanels;
o Have the button be the target of an UpdatePanel's PostBackTrigger; or
<asp:PostBackTrigger ControlID="UploadButton" />
o Call ScriptManager.RegisterPostBackControl(<Button Control>) on it during page load.
· Have a dedicated file upload page that doesn't have any UpdatePanels. Many web sites already do this anyway.
Thanks & Regards,
Arun Manglick || Tech Lead