How to add correct Content-Type when uploading file with Wininet? (HTTP PUT)

I am uploading a file with Wininet to a Sharepoint server with HTTP PUT. However, even when I specify a file name with .xlsx extension, I got on the Sharepoint server a file which says it's named .xlsx, but when downloading it, it gets a .zipextension. Also, on Sharepoint, the file does not get the little Excel icon next to it, but a more generic icon. I have tried every combination of setting the Content-type ("mime-type") with HttpAddRequestHeaders and at HttpSendRequest I could come up with.

The code below uploads the file, but Sharepoint gets the content-type wrong:

static int upload_file_to_sharepoint(LPCSTR filename, LPCSTR server, LPCSTR location)
{
    HINTERNET hIntrn = InternetOpenA(
        "magic", 
         INTERNET_OPEN_TYPE_PRECONFIG_WITH_NO_AUTOPROXY,
         NULL,
         NULL,
         0
    );
    if (!hIntrn)
        return printf("No Internet connection: %li.\n", GetLastError());

    HINTERNET hConn = InternetConnectA(
        hIntrn, 
        server,
        INTERNET_DEFAULT_HTTPS_PORT,
        NULL,
        NULL,
        INTERNET_SERVICE_HTTP,
        0,
        NULL
    );

    if (!hConn)
        return printf("Connection to update server failed: %li.\n", GetLastError());

    DWORD dwOpenRequestFlags =
        INTERNET_FLAG_KEEP_CONNECTION |
        INTERNET_FLAG_NO_COOKIES |
        INTERNET_FLAG_NO_CACHE_WRITE |
        INTERNET_FLAG_NO_UI |
        INTERNET_FLAG_RELOAD;

    PCSTR rgpszAcceptTypes[] = {
        "text/*",
        "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
        NULL
    };

    HINTERNET hReq = HttpOpenRequestA(
        hConn,
        "PUT",
        location,
        "HTTP/1.1",
        NULL,
        rgpszAcceptTypes,
        dwOpenRequestFlags,
        NULL
    );


    HANDLE hFile = CreateFileA(
        filename,
        GENERIC_READ,
        FILE_SHARE_READ,
        NULL,
        OPEN_EXISTING,
        FILE_ATTRIBUTE_NORMAL,
        NULL
    );

    if (NULL == hFile) {
        ExitProcess(1);
    }

    HANDLE hMap = CreateFileMapping(hFile, NULL, PAGE_READONLY, 0, 0, NULL);
    if (NULL == hMap) {
        ExitProcess(1);
    }

    LPVOID lpvFile = MapViewOfFile(hMap, FILE_MAP_READ, 0, 0, 0);

    DWORD dwFileSize = GetFileSize(hFile, NULL);

    CHAR mimetype[1024];
    sprintf(
        mimetype,
        "Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
    //"Content-Type: application/vnd.ms-excel\r\n"
    );

    if (!HttpAddRequestHeadersA(hReq, mimetype, -1, HTTP_ADDREQ_FLAG_REPLACE)) {
        printf("Failed adding mime header\n");
    }

    if (!HttpSendRequestA(
        hReq,
        NULL,// "Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
        0, //-1,
        lpvFile,
        dwFileSize)) {
        printf("HttpSendRequest failed: %li.\n", GetLastError());
    }

    UnmapViewOfFile(lpvFile);
    CloseHandle(hMap);
    CloseHandle(hFile);

    printf("Uploaded file to http://%s%s\n", server, location);

    return 0;
}

I captured the headers with Fiddler, got this:

PUT http://x.com/CENSORED/a.xslx HTTP/1.1
Accept: text/*, application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
Content-Disposition: attachment; filename="a.xlsx"
Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
User-Agent: magic
Host: censored
Content-Length: 772303
Connection: Keep-Alive
Pragma: no-cache


HTTP/1.1 200 OK
Cache-Control: private,max-age=0
Content-Length: 0
Expires: Tue, 15 Sep 2015 08:43:16 GMT
Last-Modified: Wed, 30 Sep 2015 08:43:16 GMT
ETag: "{0DC262D0-83AE-489A-90CD-EB23B284A3B3},14"
Server: Microsoft-IIS/7.5
SPRequestGuid: 6aba173e-41db-4b14-b7df-7714c54db282
X-SharePointHealthScore: 0
ResourceTag: rt:0DC262D0-83AE-489A-90CD-EB23B284A3B3@00000000014
Public-Extension: http://schemas.microsoft.com/repl-2
X-Powered-By: ASP.NET
MicrosoftSharePointTeamServices: 14.0.0.7145
X-MS-InvokeApp: 1; RequireReadOnly
ServerName: Xcensored
Date: Wed, 30 Sep 2015 08:43:16 GMT