Fix absolute path checking on windows (#11570)

Emby.Server.Implementations/IO/ManagedFileSystem.cs

@@ -80,12 +80,14 @@ namespace Emby.Server.Implementations.IO
         public virtual string MakeAbsolutePath(string folderPath, string filePath)
         {
             // path is actually a stream
-            if (string.IsNullOrWhiteSpace(filePath) || filePath.Contains("://", StringComparison.Ordinal))
+            if (string.IsNullOrWhiteSpace(filePath))
             {
                 return filePath;
             }
 
-            if (filePath.Length > 3 && filePath[1] == ':' && filePath[2] == '/')
+            var isAbsolutePath = Path.IsPathRooted(filePath) && (!OperatingSystem.IsWindows() || filePath[0] != '\\');
+
+            if (isAbsolutePath)
             {
                 // absolute local path
                 return filePath;
@@ -97,17 +99,10 @@ namespace Emby.Server.Implementations.IO
                 return filePath;
             }
 
-            var firstChar = filePath[0];
-            if (firstChar == '/')
-            {
-                // for this we don't really know
-                return filePath;
-            }
-
             var filePathSpan = filePath.AsSpan();
 
-            // relative path
-            if (firstChar == '\\')
+            // relative path on windows
+            if (filePath[0] == '\\')
             {
                 filePathSpan = filePathSpan.Slice(1);
             }