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I think what you pointed out is right but I cannot reproduce the ArgumentError by uploading files or running the test. The following code also doesn't cause errors.

File.open('/tmp/test.txt', flags: File::CREAT | File::EXCL | File::BINARY | File::WRONLY, binmode: true) {|io| io.puts 'hello'}

Could you let me know how I can reproduce the error?

your example fails as-well, but it depends on the platform
on Windows

File::BINARY
=> 32768

on Linux

File::BINARY
=> 0

which means it actually has no effect on Linux and it won't break if you specify the binmode argument twice.

this is because *nix operating systems don't do automatic linefeed conversion for I/O on "text" files so O_TEXT and O_BINARY flags wouldn't make sense.

I removed the flag instead, because binmode also affects encoding on Linux. This passes on both platforms.

diff --git a/app/models/attachment.rb b/app/models/attachment.rb
index c65c56722..8fba26eff 100644
--- a/app/models/attachment.rb
+++ b/app/models/attachment.rb
@@ -571,7 +571,7 @@ class Attachment < ActiveRecord::Base
         name = "#{timestamp}_#{ascii}" 
         File.open(
           File.join(path, name),
-          flags: File::CREAT | File::EXCL | File::BINARY | File::WRONLY,
+          flags: File::CREAT | File::EXCL | File::WRONLY,
           binmode: true,
           &block
         )