FIX: best effort at returning correct mime types in manifest.json

app/controllers/metadata_controller.rb

@@ -30,7 +30,7 @@ class MetadataController < ApplicationController
         {
           src: logo,
           sizes: "512x512",
-          type: "image/png"
+          type: guess_mime(logo)
         }
       ]
     }
@@ -49,4 +49,11 @@ class MetadataController < ApplicationController
 
     manifest
   end
+
+  def guess_mime(filename)
+    extension = filename.split(".").last
+    valid_image_mimes = { png: "image/png", jpg: "image/jpeg", jpeg: "image/jpeg", gif: "image/gif", ico: "image/x-icon" }
+    valid_image_mimes[extension.to_sym] || "image/png"
+  end
+
 end

spec/controllers/metadata_controller_spec.rb

@@ -15,6 +15,20 @@ RSpec.describe MetadataController do
       expect(manifest["name"]).to eq(title)
       expect(manifest["icons"].first["src"]).to eq("http://big.square/png")
     end
+
+    it 'can guess mime types' do
+      SiteSetting.large_icon_url = "http://big.square/ico.ico"
+      get :manifest
+      manifest = JSON.parse(response.body)
+      expect(manifest["icons"].first["type"]).to eq("image/x-icon")
+    end
+
+    it 'defaults to png' do
+      SiteSetting.large_icon_url = "http://big.square/noidea.bogus"
+      get :manifest
+      manifest = JSON.parse(response.body)
+      expect(manifest["icons"].first["type"]).to eq("image/png")
+    end
   end
 
   describe 'opensearch.xml' do