Sichere Bild-Uploads in ASP.NET MVC 5

[HttpPost]
public ActionResult Upload()
{
    try
    {
        // Not action parameters: MVC binds HttpPostedFileBase from the *validated*
        // Request.Files even under [ValidateInput(false)], so a file named "<b>.png"
        // would fail with an HTML 500 before this code runs.
        HttpPostedFileBase file = Request.Unvalidated.Files["file"];
        string folder = Request.Unvalidated.Form["folder"];
        string url = RteFiles.SaveUpload(HttpContext, file, folder);
        return JsonStatus(200, new { url = url });
    }
    catch (RteException ex)
    {
        return JsonStatus(ex.Status, new { error = ex.Message });
    }
}

StorageRoot root = GetRoot(context);   // 401 here first in per-user mode
if (file == null || file.ContentLength == 0)
    throw new RteException(400, "No file was uploaded.");

// FileName is whatever the browser sent (old IE sends a full path). Don't feed it
// to Path.GetFileName: on .NET Framework that throws on characters like " < > |.
string clientName = file.FileName ?? "";
clientName = clientName.Substring(Math.Max(clientName.LastIndexOf('/'), clientName.LastIndexOf('\\')) + 1);
int dot = clientName.LastIndexOf('.');
string ext = dot >= 0 ? clientName.Substring(dot).ToLowerInvariant() : "";
if (Array.IndexOf(ImageExtensions, ext) < 0)
    throw new RteException(400, "Only .jpg, .jpeg, .png, .gif and .webp images can be uploaded.");
if (file.ContentLength > MaxUploadBytes)
    throw new RteException(413, "The image is larger than the 4 MB (4,000,000 bytes) limit.");

string dir = ResolveFolder(root, folder ?? "");

byte[] data = new byte[file.ContentLength];
Stream input = file.InputStream;
int read = 0;
while (read < data.Length)
{
    int n = input.Read(data, read, data.Length - read);
    if (n <= 0) break;
    read += n;
}
// file.ContentType is chosen by the browser (or the attacker), so look at the bytes.
if (read != data.Length || !SignatureMatches(ext, data))
    throw new RteException(400, "The file is not a valid " + ext.Substring(1).ToUpperInvariant() + " image.");

// Turns a virtual folder ("", "products", "products/2026") into a directory inside the
// root. Every segment must be a valid folder name, which already rules out "..", ".",
// empty segments, "\", ":", drive letters, NUL and leading "/". The resolved path is
// then checked against the root anyway, so a mistake in the rules cannot escape it.
static string ResolveFolder(StorageRoot root, string folder)
{
    if (folder.Length > 0)
    {
        foreach (string segment in folder.Split('/'))
        {
            if (!IsValidName(segment))
                throw new RteException(400, "Invalid folder.");
        }
    }

    string full = Path.GetFullPath(Path.Combine(root.Dir, folder.Replace('/', '\\')));
    if (!(full.TrimEnd('\\') + "\\").StartsWith(root.Dir, StringComparison.OrdinalIgnoreCase))
        throw new RteException(400, "Invalid folder.");
    if (!Directory.Exists(full))
        throw new RteException(404, "Folder not found.");
    return full;
}

<!-- Uploaded files are served as static files and nothing else. Clearing the handler list
     means that even if a .aspx, .ashx or .config ever lands in uploads/, IIS will not run
     it - it gets 404, not executed. The extension allow-list also refuses to serve
     anything that is not an image (.html, .svg, .txt ...), wherever it came from. -->
<location path="uploads">
  <system.webServer>
    <handlers>
      <clear />
      <add name="StaticFile" path="*" verb="GET,HEAD" modules="StaticFileModule" resourceType="File" requireAccess="Read" />
    </handlers>
    <security>
      <requestFiltering>
        <fileExtensions allowUnlisted="false">
          <add fileExtension=".jpg" allowed="true" />
          <add fileExtension=".jpeg" allowed="true" />
          <add fileExtension=".png" allowed="true" />
          <add fileExtension=".gif" allowed="true" />
          <add fileExtension=".webp" allowed="true" />
        </fileExtensions>
      </requestFiltering>
    </security>
    <httpProtocol>
      <customHeaders>
        <add name="X-Content-Type-Options" value="nosniff" />
      </customHeaders>
    </httpProtocol>
  </system.webServer>
</location>

echo "<script>alert(1)</script>" > fake.png
curl -F "file=@fake.png" http://localhost:5402/rte/upload
{"error":"The file is not a valid PNG image."}

curl -F "file=@fake.png;filename=x.svg" http://localhost:5402/rte/upload
{"error":"Only .jpg, .jpeg, .png, .gif and .webp images can be uploaded."}