Getting StatusCode 415 “Unsupported file type” when sending a PDF in an HTTP request via Mirth Connect

I am using Mirth Connect to automate healthcare data processes, and part of my task involves sending a PDF file over HTTP. I receive the PDF as a base64 string, which I then convert into a file and send as part of an HTTP request. However, I keep receiving a 415 error code with the message that the PDF attachment file type is unsupported. Here is the error response I receive:

{
    "StatusCode": 415,
    "Message": "Unsupported tempFile8297956747860069271.pdf attachment file",
    "Success": false,
    "UserMessage": "The tempFile8297956747860069271.pdf attachment is unsupported, please select PDF files only.",
    "MemberValidation": null,
    "Error": []
}

I’ve checked that the base64 string represents a valid PDF file by converting it manually. I am unsure why the server does not recognize it as a valid PDF. Has anyone encountered this issue, or can anyone suggest what might be causing this error and how to resolve it?

    function base64ToBytes(base64String) {
        var decoder = java.util.Base64.getDecoder();
        return decoder.decode(base64String);
    }

    function writeBytesToFile(bytes) {

        var tempFile = java.io.File.createTempFile("tempFile",".pdf");
        var outputStream = new java.io.FileOutputStream(tempFile);
        outputStream.write(bytes);
        outputStream.close();

        return tempFile;
    }

    function postFileToUrl(file) {

        var url = "https://....";
        var username = $('USERNAME'); 
        var password = $('PASSWORD'); 

        var httpClient = new org.apache.commons.httpclient.HttpClient();
        var postMethod = new org.apache.commons.httpclient.methods.PostMethod(url);

       
        postMethod.setRequestHeader("Username", username);
        postMethod.setRequestHeader("Password", password);
    
        var filePart = new org.apache.commons.httpclient.methods.multipart.FilePart("file",file.getName(),file,"application/pdf","UTF-8");
    
        filePart.setContentType("application/pdf"); 
        var multipart = new org.apache.commons.httpclient.methods.multipart.MultipartRequestEntity([filePart],postMethod.getParams());

        postMethod.setRequestEntity(multipart);

        var responseCode = httpClient.executeMethod(postMethod);
        var responseBody = postMethod.getResponseBodyAsString();

        if (responseCode == 200) {
            logger.info("File uploaded successfully. Response: " + responseBody);
        } else {
            logger.error("Failed to upload file. Response: " + responseBody);
        }
        return responseBody;
    }

    var base64String = $('file');
    var bytes = base64ToBytes(base64String);
    var tempFile = writeBytesToFile(bytes);
    response = postFileToUrl(tempFile)
    tempFile.delete();

    return response;