FileController fix

- using PhysicalFile framework method (current implementation causes file locks and 500 error at heavy load)
- Add correct mimetype to header based on file extension

Oqtane.Server/Extensions/StringExtensions.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Linq;
+using Microsoft.AspNetCore.StaticFiles;
 
 namespace Oqtane.Extensions
 {
@@ -11,7 +12,20 @@ namespace Oqtane.Extensions
             {
                 return false;
             }
+
             return list.Any(f => s.StartsWith(f));
         }
+
+        public static string GetMimeType(this string fileName)
+        {
+            var provider = new FileExtensionContentTypeProvider();
+
+            if (!provider.TryGetContentType(fileName, out var contentType))
+            {
+                contentType = "application/octet-stream";
+            }
+
+            return contentType;
+        }
     }
 }