Add a rake task to fix uploads with wrong extension.

lib/tasks/uploads.rake

@@ -710,3 +710,7 @@ task "uploads:analyze", [:cache_path, :limit] => :environment do |_, args|
   puts "List of file paths @ #{path}"
   puts "Duration: #{Time.zone.now - now} seconds"
 end
+
+task "uploads:fix_incorrect_extensions" => :environment do
+  UploadFixer.fix_extensions
+end

lib/upload_fixer.rb

@@ -0,0 +1,81 @@
+class UploadFixer
+  def self.fix_extensions
+    Upload.where("uploads.extension IS NOT NULL").find_each do |upload|
+      is_external = Discourse.store.external?
+      previous_url = upload.url.dup
+
+      source =
+        if is_external
+          "https:#{previous_url}"
+        else
+          Discourse.store.path_for(upload)
+        end
+
+      correct_extension = FastImage.type(source).to_s.downcase
+      current_extension = upload.extension.to_s.downcase
+
+      if correct_extension.present?
+        correct_extension = 'jpg' if correct_extension == 'jpeg'
+        current_extension = 'jpg' if current_extension == 'jpeg'
+
+        if correct_extension != current_extension
+          new_filename = change_extension(
+            upload.original_filename,
+            correct_extension
+          )
+
+          new_url = change_extension(previous_url, correct_extension)
+
+          if is_external
+            new_url = "/#{new_url}"
+            source = Discourse.store.get_path_for_upload(upload)
+            destination = change_extension(source, correct_extension)
+
+            Discourse.store.copy_file(
+              previous_url,
+              source,
+              destination
+            )
+
+            upload.update!(
+              original_filename: new_filename,
+              url: new_url,
+              extension: correct_extension
+            )
+
+            DbHelper.remap(previous_url, upload.url)
+            Discourse.store.remove_file(previous_url, source)
+          else
+            destination = change_extension(source, correct_extension)
+            FileUtils.copy(source, destination)
+
+            upload.update!(
+              original_filename: new_filename,
+              url: new_url,
+              extension: correct_extension
+            )
+
+            DbHelper.remap(previous_url, upload.url)
+
+            tombstone_path = source.sub("/uploads/", "/uploads/tombstone/")
+            FileUtils.mkdir_p(File.dirname(tombstone_path))
+
+            FileUtils.move(
+              source,
+              tombstone_path
+            )
+          end
+        end
+      end
+    end
+  end
+
+  private
+
+  def self.change_extension(path, extension)
+    pathname = Pathname.new(path)
+    dirname = pathname.dirname.to_s != "." ? "#{pathname.dirname}/" : ""
+    basename = File.basename(path, File.extname(path))
+    "#{dirname}#{basename}.#{extension}"
+  end
+end