media assets: add dynamically-generated thumbnails (owner-only).

Add ability to dynamically generate thumbnails with:

* https://danbooru.donmai.us/media_assets/6961761.jpg?width=180&height=180

This is currently restricted to the Owner-level user because it's slow.

app/controllers/media_assets_controller.rb

@@ -15,6 +15,22 @@ class MediaAssetsController < ApplicationController
   def show
     @media_asset = authorize MediaAsset.find(params[:id])
     @post = Post.find_by_md5(@media_asset.md5)
-    respond_with(@media_asset)
+
+    if CurrentUser.is_owner? && request.format.symbol.in?(%i[jpeg webp avif])
+      width = params.fetch(:width, @media_asset.image_width).to_i
+      height = params.fetch(:height, @media_asset.image_height).to_i
+      quality = params.fetch(:quality, 85).to_i
+      original_file = @media_asset.variant(:original).open_file
+
+      if width != @media_asset.image_width || height != @media_asset.image_height || request.format != @media_asset.mime_type
+        media_file = original_file.preview!(width, height, format: request.format.symbol, quality: quality)
+      else
+        media_file = original_file
+      end
+
+      send_file(media_file.path, type: media_file.mime_type, disposition: "inline")
+    else
+      respond_with(@media_asset)
+    end
   end
 end

app/logical/media_file.rb

@@ -147,6 +147,10 @@ class MediaFile
     ExifTool.new(file).metadata
   end
 
+  def mime_type
+    Mime::Type.lookup_by_extension(file_ext)
+  end
+
   # @return [Boolean] True if the file is supported by Danbooru. Certain files may be unsupported because they use features we don't support.
   def is_supported?
     true
@@ -251,8 +255,9 @@ class MediaFile
       path: path,
       width: width,
       height: height,
-      file_ext: file_ext,
       file_size: file_size,
+      file_ext: file_ext,
+      mime_type: mime_type,
       md5: md5,
       is_corrupt?: is_corrupt?,
       is_supported?: is_supported?,
@@ -276,5 +281,5 @@ class MediaFile
     end
   end
 
-  memoize :file_ext, :file_size, :md5, :metadata
+  memoize :file_ext, :file_size, :md5, :metadata, :mime_type
 end

app/models/media_asset.rb

@@ -304,6 +304,11 @@ class MediaAsset < ApplicationRecord
       end
     end
 
+    # @return [Mime::Type] The file's MIME type.
+    def mime_type
+      Mime::Type.lookup_by_extension(file_ext)
+    end
+
     def file=(file_or_path)
       media_file = file_or_path.is_a?(MediaFile) ? file_or_path : MediaFile.open(file_or_path)