string tmpFieldName= tmpSPList.Fields.Add(dic.Key, SPFieldType.Text, false);

var newField = tmpSPList.Fields.GetFieldByInternalName(tmpFieldName);

newField.SchemaXml = newField.SchemaXml.Replace("Text", "XAttachmentField");

newField.SetCustomProperty("SiteURL", theSiteUrl);

newField.SetCustomProperty("WebURL", theWebURL);

newField.Update();