Change content-type to be always computed from file data (#14452)

* Change content-type to be always computed from file data

Restore previous behavior, detecting the content-type isn't very
expensive, and some instances may serve files as application/octet-stream
regardless of their true type, making fetching media from them fail, while
it used to work pre-3.2.0.

* Add test

spec/lib/activitypub/activity/create_spec.rb

@@ -18,6 +18,7 @@ RSpec.describe ActivityPub::Activity::Create do
 
     stub_request(:get, 'http://example.com/attachment.png').to_return(request_fixture('avatar.txt'))
     stub_request(:get, 'http://example.com/emoji.png').to_return(body: attachment_fixture('emojo.png'))
+    stub_request(:get, 'http://example.com/emojib.png').to_return(body: attachment_fixture('emojo.png'), headers: { 'Content-Type' => 'application/octet-stream' })
   end
 
   describe '#perform' do
@@ -451,6 +452,32 @@ RSpec.describe ActivityPub::Activity::Create do
         end
       end
 
+      context 'with emojis served with invalid content-type' do
+        let(:object_json) do
+          {
+            id: [ActivityPub::TagManager.instance.uri_for(sender), '#bar'].join,
+            type: 'Note',
+            content: 'Lorem ipsum :tinkong:',
+            tag: [
+              {
+                type: 'Emoji',
+                icon: {
+                  url: 'http://example.com/emojib.png',
+                },
+                name: 'tinkong',
+              },
+            ],
+          }
+        end
+
+        it 'creates status' do
+          status = sender.statuses.first
+
+          expect(status).to_not be_nil
+          expect(status.emojis.map(&:shortcode)).to include('tinkong')
+        end
+      end
+
       context 'with emojis missing name' do
         let(:object_json) do
           {