Generate PDF and attach to Gmail:MIME Multipart request flutter web app - flutter

Question

The requirement is to generate a receipt in the form of pdf and send an email to the user using Gmail API. I am able to send attachment but not attachment and message together (multipart). Here is the dart code written for a web app. If I add base64Str - i.e. bytes generated from pdf in the contentMultiPart, it simply creates pdf of base64 string. Hence I am attaching to body externally. Please help with Multipart MIME message.
class AdminCredentials {
final String? email;
final String? accessToken;
final String? idToken;
AdminCredentials(this.email, this.accessToken, {this.idToken});
}
generatepdf() async {
final pdf = pw.Document();
pdf.addPage(pw.Page(
pageFormat: PdfPageFormat.a4,
build: (pw.Context context) {
return pw.Center(
child: pw.Text("Hello World"),
); // Center
})); // Page
final bytes = await pdf.save();
return bytes;
}
sendEmail(AdminCredentials? adminCredentials) async {
Map<String, String> header = {};
header['Authorization'] = 'Bearer ${adminCredentials!.accessToken}';
header['Accept'] = 'application/json';
header['Content-type'] = 'application/json';
var pdfbytes = await generatepdf();
final base64Str = base64Encode(pdfbytes);
var from = adminCredentials.email;
var to = "test#gmail.com";
var subject ='Receipt';
var message = '<h1>Receipt generated against your flat </h1>\n<p>Dear Sir/Madam,<br/><br/>Thanks for your payment. Please find attached the receipt generated against your flat <br/><br/>Thanks and regards<br/>Office</p>';
var contentMultiPart = '''
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Type: multipart/mixed; charset="UTF-8"; boundary="part";
to: ${to}
from: ${from}
subject: ${subject}
--part
Content-Type: application/pdf; charset="base64"; Content-Disposition=application; name=receipt.pdf;
--part
Content-Type: text/html; charset="us-ascii";
$message
--part--
''';
List<int> bytes = utf8.encode(contentMultiPart);
String base64 = base64Encode(bytes);
var body = json.encode({'raw': base64+base64Str});
String apiUrl = 'https://www.googleapis.com/gmail/v1/users/' + from! + '/messages/send?uploadType=multipart';
Uri uri = Uri.parse(apiUrl);
final http.Response response = await http.post(uri, headers: header, body: body);
debugPrint('Response send');
print(json.decode(response.body));
if (response.statusCode != 200) {
final Map<String, dynamic> data = json.decode(response.body);
print('error: ' + response.statusCode.toString());
print(data);
return false;
} else {
print('ok: ' + response.statusCode.toString());
print('ok: ' + response.headers.toString());
return true;
}
}