Rails4: How to permit a hash with dynamic keys in params?

I make a http put request with following parameters:

{"post"=>{"files"=>{"file1"=>"file_content_1", "file2"=>"file_content_2"}}, "id"=>"4"}

and i need to permit hash array in my code. based on manuals I've tried like these:

> params.require(:post).permit(:files) # does not work
> params.require(:post).permit(:files => {}) # does not work, empty hash as result
> params.require(:post).permit! # works, but all params are enabled

How to make it correctly?

UPD1: file1, file2 - are dynamic keys

Rails 5.1+

params.require(:post).permit(:files => {})

Rails 5

params.require(:post).tap do |whitelisted|
  whitelisted[:files] = params[:post][:files].permit!
end

Rails 4 and below

params.require(:post).tap do |whitelisted|
  whitelisted[:files] = params[:post][:files]
end

In rails 5.1.2, this works now:

params.require(:post).permit(:files => {})

See https://github.com/rails/rails/commit/e86524c0c5a26ceec92895c830d1355ae47a7034

I understand that this is an old post. However, a Google search brought me to this result, and I wanted to share my findings:

Here is an alternative solution that I have found that works (Rails 4):

params = ActionController::Parameters.new({"post"=>{"files"=>{"file1"=>"file_content_1", "file2"=>"file_content_2"}}, "id"=>"4"})
params.require(:post).permit(files: params[:post][:files].keys)
# Returns: {"files"=>{"file1"=>"file_content_1", "file2"=>"file_content_2"}}

The difference between this answer and the accepted answer, is that this solution restricts the parameter to only 1 level of dynamic keys. The accepted answer permits multiple depths.

[Edit] Useful tip from comment

"Oh, and you need to verify that params[:post][.files] exists otherwise keys will fail"