how to allow multiple async routes in express.js - node.js

Question

I'm fairly new to Node and Express and am struggling with creating a route that takes a user uploaded file and processes it into another file. The problem is that the second time the user sends a request, I am having to wait for the first process to complete to allow the user to upload a new file.
The basic structure of route that I have is below
THE PROBLEM is that the function convertFile below is a time taking process and it keeps the server busy from accepting new requests. How do I make it so that once the project is saved in mongo db at newProject.save() - the process to convertFile runs in the background while the server accepts new requests from the same route?
I'm sending a response back the user after newProject.save() and was hoping that would allow the user to send another request. And although this sends another request, the server doesn't accept it since its busy with the previous process to convertFile
router.post('/', upload.fields([{ name: 'zip' }]), async (req, res, next) => {
let data = {
title: req.body.title,
description: req.body.description,
}
if (req.files && req.files.zip) {
const newProject = new MongoProject(data);
newProject.save()
.then(async (project) => {
res.send(project);
console.log("Project created");
const uploadedFilePath = path.normalize(req.files.zip[0].path);
// below method - "convertFile" is a time taking method
const extractZipinfo = await convertFile(uploadedFilePath , data.masterFile).then((zipInfo) => {
console.log({ zipInfo })
data.zipInfo = {
sizes: zipInfo.sizes
}
})
})
.catch(err => {
console.log("Error creating project");
return res.send(err);
})
}
})
Below is the simplified version of code in convertFile function (code modified for brevity):
I know that this can be improvised a lot, but i'm struggling with getting it to function as expected first (allowing multiple routes)
async function convertFile(inputFilePath, outputInfo) {
const outputFilePath = "output.abc";
const jsonFilePath = "output.json";
const doc = new Document(); // this is a class to store all data of the output file that we will write at the end
const _FileAPI = new fileAPI();
const outputFinalData = await _FileAPI.Init() // this is an async method
.then(() => {
const dataClass = initiateClass(); // this is a class to store data in JSON format
const paragraphs = _FileAPI.GetallParagraphs(inputFilePath);
for (let i = 0, len = paragraphs.size(); i < len; i++) {
for (let j = 0, lenj = paragraphs.size(); j < lenj; j++) {
const para = paragraphs.get(j);
// read each para and Capitalize each word
dataClass.paragraphs.push(para);
}
}
fs.writeFileSync(jsonFilePath, JSON.stringify(dataClass, null, 2), 'utf-8');
console.log("then")
}).then(() => {
const io = new NodeIO(); // this class helps in writing the file in the desired output format
const outData = io.write(outputFilePath, doc).then(() => {
outputInfo.sizes.push(fs.statSync(outputFilePath).size);
return outputInfo;
});
return outData;
});
return outputFinalData;
}