el-upload对上传图片的大小格式以及尺寸进行限制

beforeUpload(file) {this.loading = Loading.service({lock: true,text: '加载中...',background: 'rgba(255, 255, 255, 0.5)'})const isPNG = file.type === 'image/png'const isLt2M = file.size / 1024 / 1024 < 3if (!isPNG) {this.$message.error('上传图片只能是png格式!')return false}if (!isLt2M) {this.$message({message: '上传文件大小不能超过3M!',type: 'error'})return false}const isSize = new Promise((resolve, reject) => {const width = 800const height = 800const _URL = window.URL || window.webkitURLconst img = new Image()img.onload = () => {const valid = img.width === width && img.height === heightvalid ? resolve() : reject()}img.src = _URL.createObjectURL(file)}).then(() => {return file},() => {this.$message.error('图片尺寸限制为800 x 800')return Promise.reject()})return isPNG && isLt2M && isSize},