I tried your code, and it works fine.
Problem is, I have this code in a webservice and I get the infile as a base64-string, hnadles it with a memorystream and returns a base64-string. (My code is attached below) Earlier today I didn't change my code to use OutlookMessageFormat.Unicode, but now with my code below I get a msg-file with correct subject and body, but the attachment is corrupted.
Private Function ConvertEmlToMsg(ByVal strBase64 As String) As String
Dim bArrIn() As Byte = Convert.FromBase64String(strBase64)
Dim inStream As Stream = New MemoryStream(bArrIn)
Dim license As Aspose.Network.License = New Aspose.Network.License
Dim eml As MailMessage = MailMessage.Load(inStream, MessageFormat.Eml)
Dim msg As MapiMessage = MapiMessage.FromMailMessage(eml, OutlookMessageFormat.Unicode)
Dim outStream As New MemoryStream
outStream.Position = 0
Dim bArrOut() As Byte
bArrOut = outStream.ToArray