discourse/app/assets/javascripts/admin/addon/templates/modal/admin-add-upload.hbs

<DModalBody @class="add-upload-modal" @title="admin.customize.theme.add_upload">
  <div class="inputs">
    <section class="field">
      <input onchange={{action "updateName"}} type="file" id="file-input" accept="*">
      <label for="file-input">{{i18n "admin.customize.theme.upload_file_tip"}}</label>
    </section>
    <section class="field">
      <label for="theme-variable-name">{{i18n "admin.customize.theme.variable_name"}}</label>
      <Input id="theme-variable-name" @value={{this.name}} />
    </section>
    {{#if this.fileSelected}}
      {{#if this.errorMessage}}
        <span class="alert alert-error">{{this.errorMessage}}</span>
      {{/if}}
    {{/if}}
  </div>
</DModalBody>

<div class="modal-footer">
  <DButton @action={{action "upload"}} @disabled={{this.disabled}} @class="btn btn-primary" @icon="upload" @label="admin.customize.theme.upload" />
  <DModalCancel @close={{route-action "closeModal"}} />
</div>