Send email attachment c# - empty path

I'm trying to send a text file via an email in Unity. I'm using a GUI selection grid so the person can pick the file to send. I can send the email and attach the file but only if I write out the full path as a string (eg. "/C/Users/...") within the SendEmailF function (see below) rather than calling a variable. If I try to call a variable (which is a string variable with the path) for some reason it doesn't seem to pass on the information - I get ArgumentException: Path is Empty error when it attempts to attach the file. Any suggestions? Thanks!

 void OnGUI(){
             // Make selection grid with a list of files
         selStrings = Directory.GetFiles(pathName, "*.txt");
         selGridInt = GUI.SelectionGrid (new Rect(200,50,400,100), selGridInt, selStrings,1)
 
         uploadFile = selStrings[selGridInt];     // this is the selected file and path to send
         }
     }
 
         public void SendEmailF(){
 
         MailMessage mail = new MailMessage ();    // create email info
 
         mail.From = new MailAddress ("myemailaddress");
         mail.To.Add ("recipientemailaddress");
         mail.Subject = "Test Email";
         mail.Body = "This is for testing";

              // This only works if I write out the full path instead of calling 'uploadFile'
         System.Net.Mail.Attachment attachment = new System.Net.Mail.Attachment (uploadFile);   
         mail.Attachments.Add (attachment);
  
         SmtpClient smtpServer = new SmtpClient ("smtp.gmail.com");
         smtpServer.Port = 587;
         smtpServer.Credentials = new System.Net.NetworkCredential ("myemailaddress", "password") as ICredentialsByHost;
         smtpServer.EnableSsl = true;
         ServicePointManager.ServerCertificateValidationCallback = delegate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors) {
             return true;
         };
         smtpServer.Send (mail);
     }