fix: Email.AttachFromFilename does not dispose stream

closes #23

src/FluentEmail.Core/Email.cs

@@ -461,7 +461,7 @@ public IFluentEmail Attach(IEnumerable<Attachment> attachments)
 
         public IFluentEmail AttachFromFilename(string filename,  string contentType = null, string attachmentName = null)
         {
-            var stream = File.OpenRead(filename);
+            var stream = new MemoryStream(File.ReadAllBytes(filename));
             Attach(new Attachment
             {
                 Data = stream,