Lightning Component for attaching files using input file component

A rough markup of this component using input type file component. Please refer the previous article to get an idea of how the helper JS and the apex class is formed.

<div class="slds-form-element">
    <span class="slds-form-element__label" id="file-selector-id">Attachment</span>
    <div class="slds-form-element__control">
        <div class="slds-file-selector slds-file-selector_files">
            <div class="slds-file-selector__dropzone">
                <input type="file" class="slds-file-selector__input slds-assistive-text" accept="image/png" id="file-upload-input-01" aria-describedby="file-selector-id" aura:id="file" onchange="{!c.showfile}"/>
                <label class="slds-file-selector__body" for="file-upload-input-01">
                    <span class="slds-file-selector__button slds-button slds-button_neutral">
                        <c:svgIcon class="slds-button__icon slds-button__icon_left" xlinkHref="/resource/slds232/assets/icons/utility-sprite/svg/symbols.svg#upload" />
                        Upload Files
                    </span>
                    <span class="slds-file-selector__text slds-medium-show">or Drop Files</span>
                </label>
            </div>
        </div>
    </div>
</div>

showfile:function(component,event,helper){
    helper.show(component,event);
    
    var f = event.target.files[0]; 
    var fileInput = component.find("file").getElement();
    var file = fileInput.files[0];
    
    var reader = new FileReader();
    reader.onloadend = function(e) {
        var contents = e.target.result;
        var base64Mark = 'base64,';
        var dataStart = contents.indexOf(base64Mark) + base64Mark.length;
        var fileContents = contents.substring(dataStart);
        
        helper.upload(component, file, encodeURIComponent(fileContents), function(answer) {
            if (answer) {
                helper.hide(component,event);
            }
            else{
            }
        });
    }
    reader.readAsDataURL(file);
}

Lightning Component for attaching files using lightning:input

<aura:component description="FileUploader" controller="FileUploadController">
    <aura:attribute name="fileToBeUploaded" type="Object[]"/>
    <lightning:spinner aura:id="mySpinner" class="slds-hide"/>
    
    <div class=" slds-box">
        <div class="slds-grid slds-wrap">
            <lightning:input aura:id="file-input" type="file"
                             files="{!v.fileToBeUploaded}"
                             onchange="{!c.onFileUploaded}"
                             accept="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
                             label="Attachment"
                             name="file" multiple="true"/>
        </div>
    </div>
</aura:component>

({
    onFileUploaded:function(component,event,helper){
        helper.show(component,event);
        var files = component.get("v.fileToBeUploaded");
        if (files && files.length > 0) {
            var file = files[0][0];
            var reader = new FileReader();
            reader.onloadend = function() {
                var dataURL = reader.result;
                var content = dataURL.match(/,(.*)$/)[1];
                helper.upload(component, file, content, function(answer) {
                    if (answer) {
                        helper.hide(component,event);
                        // Success
                    }
                    else{
                        // Failure
                    }
                });
            }
            reader.readAsDataURL(file);
        }
        else{
            helper.hide(component,event);
        }
    }
})

({
    upload: function(component, file, base64Data, callback) {
        var action = component.get("c.uploadFile");
        console.log('type: ' + file.type);
        action.setParams({
            fileName: file.name,
            base64Data: base64Data,
            contentType: file.type
        });
        action.setCallback(this, function(a) {
            var state = a.getState();
            if (state === "SUCCESS") {
                callback(a.getReturnValue());
            }
        });
        $A.enqueueAction(action);
    },
    show: function (cmp, event) {
        var spinner = cmp.find("mySpinner");
        $A.util.removeClass(spinner, "slds-hide");
        $A.util.addClass(spinner, "slds-show");
    },
    hide:function (cmp, event) {
        var spinner = cmp.find("mySpinner");
        $A.util.removeClass(spinner, "slds-show");
        $A.util.addClass(spinner, "slds-hide");
    }
})

public class FileUploadController {
    @AuraEnabled
    public static Id uploadFile(String fileName, String base64Data, String contentType) { 
        base64Data = EncodingUtil.urlDecode(base64Data, 'UTF-8');
        Attachment a = new Attachment();
        a.parentId = '0017F000007xoiVQAQ';
        a.Body = EncodingUtil.base64Decode(base64Data);
        a.Name = fileName;
        a.ContentType = contentType; 
        insert a;
        System.debug(a.Id);
        return a.Id;
    }
}

NOTE: For demo purpose, I have hardcoded the value of an account '0017F000007xoiVQAQ' in the apex class. In actual scenario, you should get this ID from the component.