Here is how to create an uploaded file in your Rails' controller spec;

{% gist lyc4n/c4bf35eba50f427b169f8724758eb0f3 users_controller_spec.rb %}

Rack::Test::UploadedFile is from the rack-test gem which is being required by Rails' actionpack gem. It is initialized with path, optional content_type which is optional(auto sets to "text/plain" when not specified), and an optional binary flag which is default to false.

Rack::Test::UploadedFile behaves similarly with ActionDispatch::Http::UploadedFile which is the actual class used when a file is uploaded in a real controller class. The former provides reader for the original_filename and the tempfile then an accessor for the content_type while the latter provides read and write options and a bit more public methods like open, read and rewind to work with the tempfile.

For more details here is the link to the rack-test repository and documentation of ActionDispatch::Http::UploadedFile.