Retrieve file attachment from MMS via ASP.NET MVC controller

The SmsRequest class that you're using comes from the Twilio helper library for ASP.NET which aims to make it easier to integrate Twilio into ASP.NET. However, the SmsRequest and other classes do not cover all possible webhook parameters. If there's parameters missing from the class, you can still retrieve the parameters manually instead of relying on MVC Model Binding.

Based on this C# sample, I created a VB.NET sample to show how to receive and save incoming MMS files:

Imports System.IO
Imports System.Net.Http
Imports System.Threading.Tasks
Imports MimeTypes
Imports Twilio.AspNet.Mvc
Imports Twilio.TwiML
Imports Twilio.TwiML.Messaging

Public Class HomeController
    Inherits TwilioController

    Shared httpClient As HttpClient = New HttpClient

    Async Function Index() As Task(Of TwiMLResult)
        Dim response As New MessagingResponse
        Dim message As New Message

        Dim numMedia = Short.Parse(If(Request.Form.Get("NumMedia"), 0))
        If numMedia = 0 Then
            response.Message("No file received.")
            Return TwiML(response)
        End If

        For mediaIndex As Integer = 0 To numMedia - 1
            Dim mediaUrl = Request.Form.Get($"MediaUrl{mediaIndex}")
            Dim contentType = Request.Form.Get($"MediaContentType{mediaIndex}")
            Dim saveFilePath = Server.MapPath(String.Format(
                "~/App_Data/{0}{1}",
                Path.GetFileName(mediaUrl),
                MimeTypeMap.GetExtension(ContentType)
            ))
            Await DownloadUrlToFileAsync(mediaUrl, saveFilePath)
        Next

        response.Message("File received.")
        Return TwiML(response)
    End Function

    Private Async Function DownloadUrlToFileAsync(mediaUrl As String, saveFilePath As String) As Task
        Dim Response = Await httpClient.GetAsync(mediaUrl)
        Dim httpStream = Await Response.Content.ReadAsStreamAsync()
        Using fileStream As Stream = IO.File.Create(saveFilePath)
            Await httpStream.CopyToAsync(fileStream)
            Await fileStream.FlushAsync()
        End Using
    End Function


End Class

You can probably simplify the code a little by assuming there's only one file going to be sent over MMS, but it's a good idea to handle other cases too.

To get the correct file extension, I'm using this MimeTypeMap library, but you can roll your own solution.

By default the files from the incoming MMS are publicly available via the MediaUrl{mediaIndex} URL, but it's a good idea to turn on Basic Auth for these media files.

If you're turning on Basic Auth for media files, you'll need to add the authentication header to the HTTP requests, like this:

Imports System.IO
Imports System.Net.Http
Imports System.Net.Http.Headers
Imports System.Threading.Tasks
Imports MimeTypes
Imports Twilio.AspNet.Mvc
Imports Twilio.TwiML
Imports Twilio.TwiML.Messaging

Public Class HomeController
    Inherits TwilioController

    Shared httpClient As HttpClient = CreateHttpClient()
    Private Shared Function CreateHttpClient() As HttpClient
        Dim client As New HttpClient
        Dim appSettings As NameValueCollection = ConfigurationManager.AppSettings
        If Boolean.Parse(If(appSettings.Get("TwilioUseBasicAuthForMedia"), False)) Then
            Dim authString = $"{appSettings.Get("TwilioAccountSid")}:{appSettings.Get("TwilioAuthToken")}"
            authString = Convert.ToBase64String(Encoding.ASCII.GetBytes(authString))
            client.DefaultRequestHeaders.Authorization = New AuthenticationHeaderValue("Basic", authString)
        End If

        Return client
    End Function

    ...

End Class

I'm retrieving the TwilioUseBasicAuthForMedia, TwilioAccountSid, and TwilioAuthToken from the Web.config appSettings.

(Make sure you don't check those secrets into source control, and use UserSecretsConfigBuilder instead, to securely set the secrets).

Here's the source code on GitHub.

You're welcome to submit a GitHub issue and/or a PR to add support for these missing parameters to the SmsRequest. Tho, it wouldn't be as easy as normal model binding because the number of parameters increases as more files are sent over MMS.