To preview the File in Salesforce Lightning Web Component, we need the Content Document Id and the Content Version Id.

Sample Lightning Web Component:

HTML:

<template>
    <lightning-card>
        <div class="slds-m-around_medium">
            <template if:true={fileName}>
                Preview Uploaded Image - 
                <a onclick={previewImage}>
                    {fileName}
                </a>
                <br/><br/>
                <img src={imageURL}/>
            </template>
            <lightning-file-upload
                label="Attach Image"
                accept={acceptedFormats}
                record-id={recordId}
                onuploadfinished={handleUploadFinished}>
            </lightning-file-upload>
        </div>
    </lightning-card>
</template>

JavaScript:

import { LightningElement, api } from 'lwc';
import { NavigationMixin } from 'lightning/navigation'

export default class AccountFileUpload extends NavigationMixin( LightningElement ) {

    @api recordId;
    fileName;
    strContentDocId;
    strContentVerId;
    imageURL;

    get acceptedFormats() {

        return [
            '.jpeg', 
            '.png'
        ];

    }

    handleUploadFinished( event ) {
        
        let uploadedFilesList = event.detail.files;
        let uploadedFile = uploadedFilesList[ 0 ];
        console.log(
            'uploadedFile is',
            JSON.stringify(
                uploadedFile
            )
        );
        this.strContentDocId = uploadedFile.documentId;
        this.fileName = uploadedFile.name;
        this.strContentVerId = uploadedFile.contentVersionId;
        this.imageURL = '/sfc/servlet.shepherd/version/'+
            'renditionDownload?rendition=' +
            'THUMB240BY180&versionId=' + 
            this.strContentVerId+
            '&operationContext=CHATTER&contentId=' + 
            this.strContentDocId;

    }

    previewImage() {

        this[ NavigationMixin.Navigate ] ( {
            type: 'standard__namedPage',
            attributes: {
                pageName: 'filePreview'
            },
            state:{ 
                selectedRecordId: this.strContentDocId
            }
        }, false );

    }

}

js-meta.xml:

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
    <apiVersion>59.0</apiVersion>
    <isExposed>true</isExposed>
    <targets>
        <target>lightning__RecordPage</target>
    </targets>
</LightningComponentBundle>

Output: