hook_file_download not triggered
hook_file_download working perfectly, List of steps which worked for me


I made File System to Private
Created a File Field in Article Content Type and marked it as Private In Settings
Uploaded a PDF File
Link to file is something link below


http://example.com/d7/system/files/**.pdf

Created a custom module with hook_file_download, When ever i access above URL and visit any page i can see drupal set message "Hi"

function drup_custom_file_download($uri) {
  drupal_set_message("Hi");
}