uploads: allow uploading multiple files from your computer at once.

Allow uploading multiple files from your computer at once.

The maximum limit is 100 files at once. There is still a 50MB size limit
that applies to the whole upload. This limit is at the Nginx level.

The upload widget no longer shows a thumbnail preview of the uploaded
file. This is because there isn't room for it in a multi-file upload,
and because the next page will show a preview anyway after the files are
uploaded.

Direct file uploads are processed synchronously, so they may be slow.

API change: the `POST /uploads` endpoint now expects the param to be
`upload[files][]`, not `upload[file]`.

app/models/upload_media_asset.rb

@@ -3,6 +3,8 @@
 class UploadMediaAsset < ApplicationRecord
   extend Memoist
 
+  attr_accessor :file
+
   belongs_to :upload
   belongs_to :media_asset, optional: true
   has_one :post, through: :media_asset
@@ -62,16 +64,22 @@ class UploadMediaAsset < ApplicationRecord
   end
 
   def async_process_upload!
-    return if file_upload?
-    ProcessUploadMediaAssetJob.perform_later(self)
+    if file.present?
+      process_upload!
+    else
+      ProcessUploadMediaAssetJob.perform_later(self)
+    end
   end
 
   def process_upload!
-    return if file_upload?
     update!(status: :processing)
 
-    strategy = Sources::Strategies.find(source_url)
-    media_file = strategy.download_file!(source_url)
+    if file.present?
+      media_file = MediaFile.open(file)
+    else
+      media_file = source_strategy.download_file!(source_url)
+    end
+
     MediaAsset.upload!(media_file) do |media_asset|
       update!(media_asset: media_asset)
     end