error when send email with attachment

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ASP.NET

I am sending email from site. This email contains an attachment.
.These attachments is file existing on folder called upload
this is the code for generate attachment list:

List _attchmentlist = new List(); 
foreach (PersonalFile row in _list) 
{ 
  string FilePath = "~/Upload/" + row.FileID; 
  string path = this.Server.MapPath(FilePath); 
  System.IO.FileInfo file = new System.IO.FileInfo(path); 
  FileStream fs = file.OpenRead(); 
  Attachment at = new Attachment(fs, row.FileName); ; 
  _attchmentlist.Add(at); 
} 

It works fine with images, but when I try to send zip file or mp3 file
it get me message "sending failed".
When I follow the error, I found the following execption
index was out of range
So, is there any limit on file size or type?