Use cms.storage.uploads.disk instead of filesystem.default

Fixes #3332.
FileUpload widget uploads file to the disk specified by default in config/filesystem.php instead of storage.uploads.disk in config/cms.php, if we use System\Models\File following the instruction in here.
Although we can still create another class extending System\Models\File or October\Rain\Database\Attach\File and use it as the model for attachOne/Many relation, System\Models\File seems to be the one that responsible to look at storage.uploads.disk in config/cms.php, because the existing methods are using storage.uploads.*.
Credit to @pikanji

modules/system/models/File.php

@@ -2,6 +2,8 @@
 
 use Url;
 use Config;
+use File as FileHelper;
+use Storage;
 use October\Rain\Database\Attach\File as FileBase;
 
 /**
@@ -56,4 +58,23 @@ public function getStorageDirectory()
             return $uploadsFolder . '/protected/';
         }
     }
+
+    /**
+     * Returns true if storage.uploads.disk in config/cms.php is "local".
+     * @return bool
+     */
+    protected function isLocalStorage()
+    {
+        return Config::get('cms.storage.uploads.disk') == 'local';
+    }
+
+    /**
+     * Copy the local file to Storage
+     * @return bool True on success, false on failure.
+     */
+    protected function copyLocalToStorage($localPath, $storagePath)
+    {
+        $disk = Storage::disk(Config::get('cms.storage.uploads.disk'));
+        return $disk->put($storagePath, FileHelper::get($localPath), ($this->isPublic()) ? 'public' : null);
+    }
 }