Fix false positives, non-.tar flagged as .tar, from SharpCompress

Technically, the magic bytes are only found in modern revisions of the format,
but I'd say >99.9% of .tar archives we'll encounter are made with either
libarchive's bsdtar or GNU Tar.

src/BizHawk.Client.Common/SharpCompressDearchivalMethod.cs

@@ -1,10 +1,14 @@
 #nullable enable
 
+using System;
 using System.Collections.Generic;
+using System.IO;
 
 using BizHawk.Common;
+using BizHawk.Common.BufferExtensions;
 
 using SharpCompress.Archives;
+using SharpCompress.Common;
 
 namespace BizHawk.Client.Common
 {
@@ -20,13 +24,29 @@ namespace BizHawk.Client.Common
 
 			try
 			{
-				using var arcTest = ArchiveFactory.Open(fileName);
+				using var arcTest = ArchiveFactory.Open(fileName); // should throw for non-archives
+				if (arcTest.Type != ArchiveType.Tar) return true; // not expecting false positives from anything but .tar for now
 			}
 			catch
 			{
 				return false;
 			}
-			return true; // no exception? good enough
+
+			// SharpCompress seems to overzealously flag files it thinks are the in original .tar format, so we'll check for false positives. This affects 0.24.0, and the latest at time of writing, 0.27.1.
+			// https://github.com/adamhathcock/sharpcompress/issues/390
+
+			using FileStream fs = new(fileName, FileMode.Open, FileAccess.Read); // initialising and using a FileStream can throw all sorts of exceptions, but I think if we've gotten to this point and the file isn't readable, it makes sense to throw --yoshi
+			if (!fs.CanRead || !fs.CanSeek || fs.Length < 512) return false;
+
+			// looking for magic bytes
+			fs.Seek(0x101, SeekOrigin.Begin);
+			var buffer = new byte[8];
+			fs.Read(buffer, 0, 8);
+			var s = buffer.BytesToHexString();
+			if (s == "7573746172003030" || s == "7573746172202000") return true; // "ustar\000" (libarchive's bsdtar) or "ustar  \0" (GNU Tar)
+
+			Console.WriteLine($"SharpCompress identified file as original .tar format, probably a false positive, ignoring. Filename: {fileName}");
+			return false;
 		}
 
 		public SharpCompressArchiveFile Construct(string path) => new(path);
@@ -35,4 +55,4 @@ namespace BizHawk.Client.Common
 
 		public IReadOnlyCollection<string> AllowedArchiveExtensions { get; } = new[] { ".zip", ".7z", ".rar", ".gz" }; // don't try any .tar.* formats, they don't work
 	}
-}
+}