Support filename downloads with special characters

[knoepdan]

FileResponse enhancments
This commit fixes an issue with the fileResponse when the fileName contains special characters (such as Umlaute).

To support special characters in a filename in asp.net, one can use the ContentDisposition.FileNameStar property (instead of FileName). Example in the C# code below:

            var response = new HttpResponseMessage(HttpStatusCode.OK);
            byte[] contents = GetFileResponse(out string filename);
            response.Content = new ByteArrayContent(contents);
            response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment");

            // fileresponse without support for special characters (e.g.: Umlaute) 
            // response.Content.Headers.ContentDisposition.FileName = filename; // filename="=?utf-8?B?w6TDpMOkMjAyMjAxMDZfMTc0ODUzLkNTVg==?="  (incorrect with special characters)

            // or with support for special characters (e.g. Umlaute)
            response.Content.Headers.ContentDisposition.FileNameStar = filename; // filename*=utf-8''%C3%A4%C3%A4%C3%A420220106_174853.CSV

            response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");      

Regarding the regex, please have a look at:

(regex used from a comment of the accepted answer since the accepted answer did have some issues. Tested quite well)

Remark: this PR is related to pull request 3867 which in its first version did not support the case when the server sent a filename with special characters but also a filename without it. This version first uses the filename with special characters and the one without as fallback (same behavior as Mozilla Firefox). See also the discussion in #3867