Add pathway to correctly handle stream-based imports which are not zip archives

osu.Game/Database/ImportTask.cs

@@ -47,10 +47,30 @@ public ImportTask(Stream stream, string filename)
         /// </summary>
         public ArchiveReader GetReader()
         {
-            if (Stream != null)
-                return new ZipArchiveReader(Stream, Path);
+            return Stream != null
+                ? getReaderFrom(Stream)
+                : getReaderFrom(Path);
+        }
 
-            return getReaderFrom(Path);
+        /// <summary>
+        /// Creates an <see cref="ArchiveReader"/> from a stream.
+        /// </summary>
+        /// <param name="stream">A seekable stream containing the archive content.</param>
+        /// <returns>A reader giving access to the archive's content.</returns>
+        private ArchiveReader getReaderFrom(Stream stream)
+        {
+            if (!(stream is MemoryStream memoryStream))
+            {
+                // This isn't used in any current path. May need to reconsider for performance reasons (ie. if we don't expect the incoming stream to be copied out).
+                byte[] buffer = new byte[stream.Length];
+                stream.Read(buffer, 0, (int)stream.Length);
+                memoryStream = new MemoryStream(buffer);
+            }
+
+            if (ZipUtils.IsZipArchive(memoryStream))
+                return new ZipArchiveReader(memoryStream, Path);
+
+            return new LegacyByteArrayReader(memoryStream.ToArray(), Path);
         }
 
         /// <summary>

osu.Game/Utils/ZipUtils.cs

@@ -9,6 +9,34 @@ namespace osu.Game.Utils
 {
     public static class ZipUtils
     {
+        public static bool IsZipArchive(Stream stream)
+        {
+            try
+            {
+                stream.Seek(0, SeekOrigin.Begin);
+
+                using (var arc = ZipArchive.Open(stream))
+                {
+                    foreach (var entry in arc.Entries)
+                    {
+                        using (entry.OpenEntryStream())
+                        {
+                        }
+                    }
+                }
+
+                return true;
+            }
+            catch (Exception)
+            {
+                return false;
+            }
+            finally
+            {
+                stream.Seek(0, SeekOrigin.Begin);
+            }
+        }
+
         public static bool IsZipArchive(string path)
         {
             if (!File.Exists(path))