When running an ASP.NET MVC 4.7 API that exports a PDF file, the application behaves correctly locally, returning the file with a Content-Type of application/pdf. However, upon deployment to a production environment, the Content-Type is unexpectedly set to text/html; charset=utf-8.

[AllowAnonymous]
[HttpGet]
public FileContentResult Export(Guid id, bool? PrintLetterOnly, bool? PrintMentions)
{
    var model = Show(id, true);
    if (model == null)
        return null;

    Response.ContentType = "application/pdf";

    Response.AddHeader("Content-Disposition", $"attachment; filename={id}.pdf");

    var pdfResult = new ViewAsPdf("export", model)
    {
        FileName = id + ".pdf",
        PageMargins = new Rotativa.Options.Margins(0, 0, 0, 0),
        PageSize = Rotativa.Options.Size.A4,
        MinimumFontSize = 20,
        ContentDisposition = ContentDisposition.Attachment
    };
    var pdfBytes = pdfResult.BuildFile(this.ControllerContext);
    return File(pdfBytes, "application/pdf", id + ".pdf");
}

This API method correctly generates a PDF using Rotativa library and sets the Content-Type to application/pdf. However, in production, despite the correct configuration, the response Content-Type unexpectedly changes to text/html; charset=utf-8, causing the browser to interpret the file as HTML rather than a PDF.

the code from java script for api calling

$("#download").click(function () {

    const myHeaders = new Headers();
    myHeaders.append("Cookie", "ASP.NET_SessionId=n1q1vjmx2noir5esjdlvjnqb");

    const requestOptions = {
        method: "GET",
        headers: myHeaders,
        redirect: "follow"
    };
    fetch("/Process/Export/@Model.Id?PrintLetterOnly=True", requestOptions)
        .then(response => {
            debugger;
            if (!response.ok) {
                throw new Error('Network response was not ok');
            }
            return response.blob();
        })
        .then(blob => {
            const url = window.URL.createObjectURL(blob);
            const a = document.createElement('a');
            a.style.display = 'none';
            a.href = url;
            a.download = "@(Model.Id).pdf"; // Specify the file name here
            document.body.appendChild(a);
            a.click();
            window.URL.revokeObjectURL(url);
        })
        .catch((error) => console.error(error))
});

I expected the issue from iis server configuration

I think the issue from server configuration or iis configuration not accept to return .pdf

I tried too much ways to convert content type from text/html; charset=utf-8 to application/pdf