AJAX :: Verify Whether File Exists When Uploading Using FileUpload Control

In my application Im saving multiple files(image,pdf,txt) to a folder.  Suppose I have an image with name "image1", if this image already exists in the folder and if another user tries to add another image with the same name "image1" then, it should give message to change the File name.

I had used below code:

string filePath = DamRep_FileUpload.PostedFile.FileName;
string filename = Path.GetFileName(filePath);
string contenttype = String.Empty;
if (contenttype != String.Empty)
{
if (File.Exists("~/FTP/File/"))

[Code] ....

but its not working.