after trying to implement a file field that users can upload a screenshot with on a feedback with paperclip I recieved some 'no handle errors'. I also made a question about my paperclip issue here.
By looking at the comments to my question it might have to do with multipart.
BUT since I don't use a rails form I can't do something like form_for multipart true. I'm handling some js via my current form and don't really want to change all of this, that why I now tried switching from deprecated paperclip to rails active storage. So far so good.
Got my form still running and works fine until I come to the point of writing the screenshot in the database.
This is a example response of what I get right now when submitting a feedback.
Started POST "/feedback" for 127.0.0.1 at 2018-10-25 12:21:13 +0200
Processing by FeedbackController#create as */*
  Parameters: {"name"=>"dunnothename", "message"=>"blablablablablabla", "topic"=>"Bug", "screenshot"=>"C:\\fakepath\\dhdr.jpeg"}
  User Load (0.5ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 ORDER BY "users"."id" ASC LIMIT $2  [["id", 1], ["LIMIT", 1]]
   (0.2ms)  BEGIN
  Feedback Create (41.3ms)  INSERT INTO "feedbacks" ("message", "topic", "name", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["message", "blablablablablabla"], ["topic", "Bug"], ["name", "dunnothename"], ["created_at", "2018-10-25 10:21:13.410630"], ["updated_at", "2018-10-25 10:21:13.410630"]]
   (0.6ms)  COMMIT
   (0.2ms)  BEGIN
   (0.2ms)  COMMIT
No template found for FeedbackController#create, rendering head :no_content
Completed 204 No Content in 1268ms (ActiveRecord: 48.1ms)
When trying to add the screenshot param to my controller-method ( @feedback = Feedback.create name: ....., screenshot: params[:screenshot]) Ill get the following response:
Started POST "/feedback" for 127.0.0.1 at 2018-10-25 12:31:34 +0200
Processing by FeedbackController#create as */*
  Parameters: {"name"=>"dunnothename", "message"=>"blablablablablabla", "topic"=>"Bug", "screenshot"=>"C:\\fakepath\\dhdr.jpeg"}
  User Load (0.7ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 ORDER BY "users"."id" ASC LIMIT $2  [["id", 1], ["LIMIT", 1]]
Completed 500 Internal Server Error in 64ms (ActiveRecord: 11.0ms)
ActiveSupport::MessageVerifier::InvalidSignature - ActiveSupport::MessageVerifier::InvalidSignature:
  app/controllers/feedback_controller.rb:6:in `create'
Feedback.rb
class Feedback < ApplicationRecord
  validates :name, presence: true, length: {minimum: 3}
  validates :message, presence: true, length: {minimum: 5}
  validates :topic, length: {minimum: 3}
  has_one_attached :screenshot
end
_feedback.html.erb
<div id="feedback-message" class="feedback-message" style="color: green;">Feedback sent</div>
<span class="label label-info">Name</span><%= text_field_tag :name, "", class: 'form-control' %>
<span class="label label-info">Topic</span>
<select name="topic" id="topic" class="form-control">
  <option value="Bug">Bug</option>
  <option value="Suggestion">Suggestion</option>
  <option value="Other">Other</option>
</select>
<span class="label label-info">Screenshot</span>
<label class="image-upload form-control">
  <i class="fa fa-cloud-download" id="upload_button"></i>
  <input id="upload_input" type="file" name="feedback[screenshot]"/>
</label>
<span class="label label-info">Message</span> <%= text_area_tag :message, "", class: 'form-control', rows: 3 %>
<hr>
<%= submit_tag 'Submit', id: 'submit-feedback', class: 'btn btn-success center' %>
<script>
    $(document).ready(function () {
        var msg = document.getElementById('feedback-message');
        var submit = $('#submit-feedback');
        submit.click(function () {
            msg.style.display = 'block';
            submit.prop('disabled', true);
            setTimeout(function () {
                submit.prop('disabled', false);
                msg.style.display = 'none';
            }, 5000);
            $.ajax({
                url: '/feedback',
                type: 'POST',
                data: {
                    authenticity_token: $('[name="authenticity_token"]').val(),
                    name: $('#name').val(),
                    message: $('#message').val(),
                    topic: $('#topic').val(),
                    screenshot: $('#upload_input').val(),
                },
            });
        });
    });
    var button = document.getElementById('upload_button');
    var input = document.getElementById('upload_input');
    var span = document.createElement('span');
    span.style.paddingLeft = '20px';
    span.style.color = 'green';
    button.appendChild(span);
    input.style.display = 'none';
    button.style.display = 'initial';
    button.addEventListener('click', function (e) {
        e.preventDefault();
        input.click();
    });
    input.addEventListener('change', function () {
        span.innerText = '' + this.value;
    });
</script>
feedback_controller.rb
class FeedbackController < ApplicationController
  def index;
  end
  def create
    @feedback = Feedback.create name: params[:name], message: params[:message], topic: params[:topic]
    @feedback.screenshot.attach(params[:screenshot])
    authorize @feedback
    if @feedback.save
      #FeedbackMailer.with(user: @user).feedback_sent.deliver_later
    end
  end
end
Update:
After trying to implement what @Niraj mentioned I get the following response:
Started POST "/feedback" for 127.0.0.1 at 2018-10-25 13:06:34 +0200
Processing by FeedbackController#create as */*
  Parameters: {"name"=>"askdalskjdajsdpojapsdo", "message"=>"asdasldkaspdkasüpdkapsod", "topic"=>"Bug", "screenshot"=>"C:\\fakepath\\dhdr.jpeg"}
  User Load (0.4ms)  SELECT  "users".* FROM "users" WHERE "users"."id" = $1 ORDER BY "users"."id" ASC LIMIT $2  [["id", 1], ["LIMIT", 1]]
   (0.2ms)  BEGIN
  Feedback Create (1.4ms)  INSERT INTO "feedbacks" ("message", "topic", "name", "created_at", "updated_at") VALUES ($1, $2, $3, $4, $5) RETURNING "id"  [["message", "asdasldkaspdkasüpdkapsod"], ["topic", "Bug"], ["name", "askdalskjdajsdpojapsdo"], ["created_at", "2018-10-25 11:06:34.518275"], ["updated_at", "2018-10-25 11:06:34.518275"]]
   (0.9ms)  COMMIT
  ActiveStorage::Attachment Load (0.3ms)  SELECT  "active_storage_attachments".* FROM "active_storage_attachments" WHERE "active_storage_attachments"."record_id" = $1 AND "active_storage_attachments"."record_type" = $2 AND "active_storage_attachments"."name" = $3 LIMIT $4  [["record_id", 3], ["record_type", "Feedback"], ["name", "screenshot"], ["LIMIT", 1]]
Completed 500 Internal Server Error in 67ms (ActiveRecord: 9.4ms)
ActiveSupport::MessageVerifier::InvalidSignature - ActiveSupport::MessageVerifier::InvalidSignature:
  app/controllers/feedback_controller.rb:7:in `create'
Update2:
class FeedbackPolicy < ApplicationPolicy
  def index?
    true
  end
  def create?
    user.present?
  end
  def new?
    user.present?
  end
  def update?
    return true if user.present?
  end
  def destroy?
    return true if user.present?
  end
  private
  def feedback
    record
  end
end