Moodle corrupted uploaded files

I am using moodle api core_files_upload with node js using this script

const { CallService } = require("../MoodleWS")

var Upload = async function(userid, file) {
    var base64 = await toBase64(file)
        .then(r => {
            return r;
        })
        .catch((e) => {
            console.log(e)
        });
    console.log(base64);
    var param = {
        itemid: 0,
        instanceid: userid,
        filearea: 'draft',
        filecontent: base64,
        component: 'user',
        filepath: '/',
        filename: file.name,
        contextlevel: 'user'
    }
    // return promise calling web service, basically returned axios
    return CallService('POST', 'core_files_upload', false, null, param);
}

const toBase64 = file => new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = () => resolve(reader.result);
    reader.onerror = () => reject(reader.error);
})

module.exports = { Upload }

the function returned success and files uploaded, I have checked the uploaded file size is the same as the original files, unfortunately I failed to open the files and keeps saying like image below

and images also can't be displayed

the uploaded base64 also have the mime/type header like data:application/pdf;base64,JVBERi0xLjQNJeL... I don't really know what went wrong. When I tried to upload files using the native web version of moodle, the files uploaded correctly. So anyone can help? Thanks

So it turned out that I don't need to include the mime/type from the base64. So I just remove the data:application/pdf;base64 and modify my param a little so it became like

var base64 = await toBase64(file)
        .then(r => {
            return r;
        })
        .catch((e) => {
            console.log(e)
        });
    var param = {
        itemid: 0,
        instanceid: userid,
        filearea: 'draft',
        filecontent: base64.split(',')[1],
        component: 'user',
        filepath: '/',
        filename: file.name,
        contextlevel: 'user'
    }