Sample Code:

Component:

<aura:component implements="force:appHostable" >   
    <lightning:fileUpload label="Attach receipt"
        name="fileUploader" 
        multiple="true" 
        accept=".pdf, .png"
        recordId="0016A00000MneVlQAJ" 
        onuploadfinished="{!c.handleUploadFinished}" />   
</aura:component>

Controller:

({
    handleUploadFinished: function (cmp, event) {
        var uploadedFiles = event.getParam("files");
        alert("Files uploaded");
    }
})

event.getParam(“files”) returns the list of files uploaded. Here recordId attribute is hard coded.

For more information, check the below link

https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/aura_compref_lightning_fileUpload.htm

Output: