How to write a Tempfile as binary - ruby-on-rails

Question

When trying to write a string / unzipped file to a Tempfile by doing:
temp_file = Tempfile.new([name, extension])
temp_file.write(unzipped_io.read)
Which throws the following error when I do this with an image:
Encoding::UndefinedConversionError - "\xFF" from ASCII-8BIT to UTF-8
When researching it I found out that this is caused because Ruby tries to write files with an encoding by default (UTF-8). But the file should be written as binary, so it ignores any file specific behavior.
Writing regular File you would be able to do this as following:
File.open('/tmp/test.jpg', 'rb') do |file|
file.write(unzipped_io.read)
end
How to do this in Tempfile

Answer 1

Tempfile.new passes options to File.open which accepts the options from IO.new, in particular:
:binmode
If the value is truth value, same as “b” in argument mode.
So to open a tempfile in binary mode, you'd use:
temp_file = Tempfile.new([name, extension], binmode: true)
temp_file.binmode? #=> true
temp_file.external_encoding #=> #<Encoding:ASCII-8BIT>
In addition, you might want to use Tempfile.create which takes a block and automatically closes and removes the file afterwards:
Tempfile.create([name, extension], binmode: true) do |temp_file|
temp_file.write(unzipped_io.read)
# ...
end

Answer 2

I have encountered the solution in an old Ruby forum post, so I thought I would share it here, making it easier for people to find:
https://www.ruby-forum.com/t/ruby-binary-temp-file/116791
Apparently Tempfile has an undocumented method binmode, which changes the writing mode to binary and thus ignoring any encoding issues:
temp_file = Tempfile.new([name, extension])
temp_file.binmode
temp_file.write(unzipped_io.read)
Thanks unknown person who mentioned it on ruby-forums.com in 2007!

Answer 3

Another alternative is IO.binwrite(path, file_content)