Use .dcstylejson instead of .dcstyle.json

app/assets/javascripts/discourse/components/json-file-uploader.js.es6

@@ -32,6 +32,10 @@ export default Em.Component.extend({
 
   }.on('didInsertElement'),
 
+  accept: function() {
+    return ".json,application/json" + (this.get('extension') ? "," + this.get('extension') : "");
+  }.property('extension'),
+
   setReady: function() {
     let parsed;
     try {

app/assets/javascripts/discourse/templates/components/json-file-uploader.hbs

@@ -1,5 +1,5 @@
 <div class="jsfu-file">
-  <input id="js-file-input" type="file" style="display:none;" accept=".json,application/json">
+  <input id="js-file-input" type="file" style="display:none;" accept={{accept}}>
   {{d-button class="fileSelect" action="selectFile" class="" icon="upload" label="upload_selector.select_file"}}
   {{conditional-loading-spinner condition=loading size="small"}}
 </div>

app/assets/javascripts/discourse/templates/modal/upload-customization.hbs

@@ -1,6 +1,6 @@
 <form {{action "dummy" on="submit"}}>
   <div class='modal-body'>
-    {{json-file-uploader value=customizationFile}}
+    {{json-file-uploader value=customizationFile extension=".dcstylejson"}}
   </div>
   <div class="modal-footer">
     {{d-button class='btn-primary' action='createCustomization' type='submit' disabled=notReady icon="plus" label='admin.customize.import'}}

app/controllers/admin/site_customizations_controller.rb

@@ -62,7 +62,7 @@ class Admin::SiteCustomizationsController < Admin::AdminController
       format.any(:html, :text) do
         raise RenderEmpty.new if request.xhr?
 
-        response.headers['Content-Disposition'] = "attachment; filename=#{@site_customization.name.parameterize}.dcstyle.json"
+        response.headers['Content-Disposition'] = "attachment; filename=#{@site_customization.name.parameterize}.dcstylejson"
         response.sending_file = true
         render json: SiteCustomizationSerializer.new(@site_customization)
       end