finalize refactoring & ready for the PR

MalwareMultiScan.Api/Attributes/MaxFileSizeAttribute.cs

@@ -0,0 +1,28 @@
+using System.ComponentModel.DataAnnotations;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace MalwareMultiScan.Api.Attributes
+{
+    /// <summary>
+    /// Validate uploaded file size for the max file size defined in settings.
+    /// </summary>
+    public class MaxFileSizeAttribute : ValidationAttribute
+    {
+        /// <inheritdoc />
+        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
+        {
+            var maxSize = validationContext
+                .GetRequiredService<IConfiguration>()
+                .GetValue<long>("MaxFileSize");
+            
+            var formFile = (IFormFile) value;
+            
+            if (formFile == null || formFile.Length > maxSize)
+                return new ValidationResult($"File exceeds the maximum size of {maxSize} bytes");
+            
+            return ValidationResult.Success;
+        }
+    }
+}