If the file was inside the Moodle files API, then it is not surprising that attempting to get the file contents via file_get_contents($imageurl) wouldn't work.
If you are accessing a file via the URL, then Moodle will apply security checks to see if the user requesting the file has access to it - file_get_contents() would not include any sort of session cookie to identify the request as coming from a logged in user, so the request would be denied.
If you are accessing a file via the URL, then Moodle will apply security checks to see if the user requesting the file has access to it - file_get_contents() would not include any sort of session cookie to identify the request as coming from a logged in user, so the request would be denied.