Add LegacyFileArchiveReader

Treats files as "archives" for import.

osu.Game/Database/ArchiveModelManager.cs

@@ -486,6 +486,8 @@ namespace osu.Game.Database
                 return new ZipArchiveReader(File.Open(path, FileMode.Open, FileAccess.Read, FileShare.Read), Path.GetFileName(path));
             if (Directory.Exists(path))
                 return new LegacyDirectoryArchiveReader(path);
+            if (File.Exists(path))
+                return new LegacyFileArchiveReader(path);
             throw new InvalidFormatException($"{path} is not a valid archive");
         }
     }

osu.Game/IO/Archives/LegacyFileArchiveReader.cs

@@ -0,0 +1,34 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System.Collections.Generic;
+using System.IO;
+
+namespace osu.Game.IO.Archives
+{
+    /// <summary>
+    /// Reads a file on disk as an archive.
+    /// Note: In this case, the file is not an extractable archive, use <see cref="ZipArchiveReader"/> instead.
+    /// </summary>
+    public class LegacyFileArchiveReader : ArchiveReader
+    {
+        private readonly string path;
+
+        public LegacyFileArchiveReader(string path)
+            : base(Path.GetFileName(path))
+        {
+            // re-get full path to standardise
+            this.path = Path.GetFullPath(path);
+        }
+
+        public override Stream GetStream(string name) => File.OpenRead(path);
+
+        public override void Dispose()
+        {
+        }
+
+        public override IEnumerable<string> Filenames => new[] { Name };
+
+        public override Stream GetUnderlyingStream() => null;
+    }
+}