to_file has been removed Show archive.org snapshot in Paperclip 3.0.1.
to_file
Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)
Instead of using File to access Paperclip storage objects (like this: File.read(file.to_file.path)) you can use
File
File.read(file.to_file.path)
Paperclip.io_adapters.for(file).read