SalesForce File (that replaces Attachments) Upload via JS example.

SalesForceFileUpload.js

/*
Uses API, should have similar before js runs;
<script>const __sfdcSessionId = '{!sessionId}'</script>
<script src="/soap/ajax/42.0/connection.js" type="text/javascript"></script>
*/
const addAttachment = (parentId, filename, file) => {

    const reader = new FileReader()

    if(file == undefined){
        mkToast('A file is required to proceed', 'warning')
        return false
    }

    if(file.size > 26214400){  //Where 26214400 is byte equivalent of 25MB
        mkToast('File size not supported (over 25MB)', 'warning')
        return false
    }

    reader.onload = (e) => {
    	// Create a document > create a new version via the ContentVersion object without setting the ContentDocumentId. 
		// This automatically creates a parent document record.
        const ContentVersion = new sforce.SObject('ContentVersion') //Initialize the contentDocumentLink
        ContentVersion.Title = filename
        ContentVersion.PathOnClient = parentId
        ContentVersion.VersionData = (new sforce.Base64Binary(e.target.result)).toString() //Document Data

        const version = sforce.connection.create([ContentVersion])
        const versionId = version[0].id

        if(versionId){

            // make soql call for ContentDocumentId with new versionId via Apex... ugh
            // return [SELECT ContentDocumentId FROM ContentVersion where Id = :versionId].ContentDocumentId;
            const getContentDocumentId = new Promise((resolve, reject) => {
                Visualforce.remoting.Manager.invokeAction(
                    '{!$RemoteAction.FormAttachments.getDocumentId}',
                    versionId,
                    (result, e) => e.status 
                        ? resolve(result) 
                        : reject(e.message)
                );
            })

            getContentDocumentId.then(ContentDocumentId => {
                // Create link
                const contentDocumentLink = new sforce.SObject('contentDocumentLink') //Initialize the contentDocumentLink
                contentDocumentLink.ContentDocumentId = ContentDocumentId //Document Id
                contentDocumentLink.LinkedEntityId = parentId //Linked Object Id / id in URL / PW.Id
                // Required! The permission granted to the user of the shared file in a library. 
                // This is determined by the permission the user already has in the library. This field is available in API version 25.0 and later.
                // V : Viewer premission. The user can explicitly view but not edit the shared file.
                // C : Collaborator permission. The user can explicitly view and edit the shared file.
                // I : Inferred permission. The user’s permission is determined by the related record. For shares with a library, this is defined by the permissions the user has in that library.
                contentDocumentLink.ShareType = 'V'

                const result = sforce.connection.create([contentDocumentLink])

                result[0].success
					? console.log('Uploaded Successfully')
                    : console.error(result[0].errors.message)
            })
            .catch(e => console.error(e))
        }
    }
	
    reader.readAsBinaryString(file) // Read file to kick things off
}