This vf page helps to upload the attachment to the web to case. So that you can embed this VF page inside your lightning component.

CaseController.apxc

public class CaseController {

    public transient Attachment objAttachment;
    
    public String message{get;set;}
    public String messageType{get;set;}
    public Boolean fileUploadProcessed{get;set;}
    public Attachment getObjAttachment () {
        objAttachment = new Attachment ();
        return objAttachment; 
    }

    
    public  void uploadFile(){
        try {
            insert objAttachment;
            message = 'File was uploaded successfully';
            messageType = 'success';
            fileUploadProcessed = true;
        }catch(Exception e){
            message = e.getMessage();
            messageType = 'error';
            System.debug('Error++:'+e);
        }
    }
}

FileUploadPage.vfp

<apex:page showHeader="false" id="UploadFilePage" sidebar="false" docType="html-5.0" lightningStylesheets="true" controller="CaseController">
    <script>
    	let parentVal;
        window.addEventListener ("message", function (event) {
            data = event.data;
            if (typeof (event.data.uploadFile) !== undefined && (event.data.uploadFile == true)) {
                document.getElementById ('{!$Component.theForm.parentIdText}').value = event.data.parentId;
                document.getElementById ('{!$Component.theForm.uploadFileButton}').click();
            }
        }, false);
        let lexOrigin = "https://{!$CurrentPage.parameters.lcHost}"
        let data;

        function sendToLC (message) {
            if (typeof (lexOrigin) !== undefined) {
                parent.postMessage (message, lexOrigin);
            }
        }
        function onFileSelected (selectedFile) {
			let MAX_FILE_SIZE  = Number ('{!$Label.MaxAttachmentSize}');
            if (selectedFile && selectedFile.files && selectedFile.files.length > 0) {
            	let file = selectedFile.files[0];
                if (file.size > MAX_FILE_SIZE) {
                    document.getElementById ('deleteBtn').style.visibility = 'hidden';
                    document.getElementById ('{!$Component.theForm.fileSelectedForUpload}').value = "";
                    sendToLC({'state': 'ONFILEOVERSIZE','message' : 'Alert : File size cannot exceed ' + ((MAX_FILE_SIZE / (1024*1024))).toFixed (2) + ' Mbytes.\n' + ' Selected file size: ' + Math.ceil((file.size / (1024*1024))) + ' Mbytes'});
                } else {
                    sendToLC({'state': 'ONFILESELECTED'});
                    document.getElementById ('deleteBtn').style.visibility = 'visible';
                }
            }
        }
    	function deleteAttachment () {
            document.getElementById ('{!$Component.theForm.fileSelectedForUpload}').value = "";
            sendToLC({'state': 'ONFILEDESELECTED'});
            document.getElementById ('deleteBtn').style.visibility = 'hidden';
        }
        sendToLC ({'state' : 'LOADED', 'vfHost' : '{!LEFT ($CurrentPage.URL, FIND ('/', $CurrentPage.URL, 9 ))}'});
    </script>
    <apex:slds />
    <div class="slds-scope" align="Center">
        <apex:form id="theForm" enctype="multipart/form-data">
            <apex:inputHidden value="{!objAttachment.ParentId}" id="parentIdText"/>            
            <apex:commandButton value="Save" action="{!uploadFile}" id="uploadFileButton" style="visibility: hidden;"/><br/>           
            <apex:inputfile id="fileSelectedForUpload" onchange="onFileSelected(this);" value="{!objAttachment.body}" filename="{!objAttachment.Name}" size="{!objAttachment.BodyLength}" contentType="{!objAttachment.ContentType}"></apex:inputfile>
            <input type="button" class="slds-button" value="Delete" onclick="deleteAttachment ()" id="deleteBtn" />
        </apex:form>

        <apex:pageBlock rendered="{!fileUploadProcessed == true}" id="theBlock">
            <script>
            var theBlock = document.getElementById('{!$Component.theBlock}');
                theBlock.style.border = 'none';
                theBlock.style.display = 'none';

                sendToLC({'state': 'fileUploadprocessed', 'message': '{!message}', 'messageType': '{!messageType}'});
            </script>
        </apex:pageBlock>
    </div>
</apex:page>