How to upload a Salesforce File using REST API properly?

I had similar issue when I was uploading a file from a Salesforce Guest Site and the below code worked for me.

LWC - HTML Code:

<label class="form-label" for="fileUpload">Please upload your invoice</label>
<lightning-input type="file" class="form-label" variant="label-hidden" accept=".png, .jpg, .jpeg, .pdf" onchange={openfileUpload}>
</lightning-input>
<p class="form-label"
   style="font-size: 11px;">Note: Maximum file size: 10MB. Accepted formats: .png, .jpg, .jpeg, .pdf
</p>
<template if:true={uploadedFileInfo}>
    <p class="form-label" style="color: darkblue;">{uploadedFileInfo.fileName}</p>
</template>

LWC - JS Code:

import createUploadedFile from '@salesforce/apex/YourApexClass.createUploadedFile';

@track uploadedFileInfo;

// Reading the uploaded file
openfileUpload(event) {
    const uploadedFile = event.target.files[0];
    let fileSize = uploadedFile.size;
    let fileType = uploadedFile.type;
    if(fileSize > 11004402){
        alert('Please reduce the file size');
    }
    else{
        var reader = new FileReader();
        reader.onload = () => {
            var base64 = reader.result.split(',')[1]; // This is file content
            this.uploadedFileInfo = {
                'fileName': uploadedFile.name,
                'base64': base64,
                'fileType'  : uploadedFile.type
            }
            console.log(this.uploadedFileInfo)
        }
        reader.readAsDataURL(uploadedFile)
    }
}

// Calling Apex method to create file
handleSubmit(event){
    let paramsToApex = {
        // Any other field values to be sent to server
        parentID    : 00QKN012345TCFU2A4,
        fileName    : (this.uploadedFileInfo != null && this.uploadedFileInfo.fileName != null) ? this.uploadedFileInfo.fileName : '#NO#FILE',
        base64Data  : (this.uploadedFileInfo != null && this.uploadedFileInfo.base64 != null) ? this.uploadedFileInfo.base64 : '#NO#FILE'
    }
    
    createUploadedFile({paramString : JSON.stringify(paramsToApex)}).then((result) =>{
        // Handle server response
    }).catch((error) =>{
        // Handle any errors
    });
}

Apex Code:

@AuraEnabled
public static String createUploadedFile(String paramString){
    Map<String, Object> paramsMap = (Map<String, Object>)JSON.deserializeUntyped(paramString);
    String parentID     = (String) paramsMap.get('parentID');
    String fileName     = (String) paramsMap.get('fileName');
    String base64Data   = (String) paramsMap.get('base64Data');
    if(!fileName.equalsIgnoreCase('#NO#FILE')){
        createContentVersion(fileName,base64Data,parentID);
    }
    return 'SUCCESS';
}


@future(callout=true)
public static void createContentVersion(String title, String versionData, Id parentId) {
    // If my org credentials are stored in Custom Settings. I recommend using Named Credentials instead
    API_Settings__c apiCredsCustSetting = API_Settings__c.getInstance('Self_Org_API_Details');
    String reasonForChange  = 'New Document';
    String endPointURL  = apiCredsCustSetting.endpoint__c; //Ex: https://abcOrg.my.salesforce.com
    endPointURL         = endPointURL+'/services/data/59.0/sobjects/ContentVersion/';

    String contentVersionBody = '{' +
            '"Title" : "' + title + '",' +
            '"VersionData" : "' + versionData + '",' +
            '"ReasonForChange" : "' + reasonForChange + '",' +
            '"PathOnClient" : "' + title + '"' +
            '}';

    HttpRequest request = new HttpRequest();
    request.setEndpoint(endPointURL);
    request.setMethod('POST');
    request.setHeader('Content-Type', 'application/json');
    request.setHeader('Authorization', 'Bearer ' + getAccessToken(apiCredsCustSetting));
    request.setBody(contentVersionBody);

    Http http=new Http();
    HttpResponse response=http.send(request);

    if (response.getStatusCode() == 201) {
        Map<String,Object>jsonResponse = (Map<String, Object>) JSON.deserializeUntyped(response.getBody());
        Boolean isSuccess = (Boolean) jsonResponse.get('success');

        if(isSuccess == true) {
            String newContentVersionId=(String) jsonResponse.get('id');

            ContentDocumentLink cdl = new ContentDocumentLink();
            cdl.ContentDocumentId = [SELECT ContentDocumentId FROM ContentVersion WHERE Id=: newContentVersionId LIMIT 1].ContentDocumentId;
            cdl.LinkedEntityId = parentId;
            cdl.ShareType = 'V';
            insert cdl;
        }
    }
    else {
        System.debug('Errored response: ' +response.getBody());
    }
}