`<u-uploader>`添加图片预览、和拖拽上传等功能

[z-ZYS-s]

Props/Attrs
url(String)
data(Object)
data-type(String)
name(String)
headers(Object)
withCredentials(Boolean)
accept/extension(String、Array)
maxSize(String、Number)
limit(String、Number)
multiple(Boolean)
directory(Boolean)
draggable(Boolean)
value.sync, v-model(Object, Array)
showFileList(Boolean)
viewMode(String)(text, image 和 image-card)
disabled(Boolean)
autoUpload(Boolean)

File对象属性
uid
name
status
url

Events
@add
@before-upload
@Upload
@progress
@complete
@success
@error
@change
@remove
@Preview
@extension-error
@exceeded-size
@exceeded-limit

Slots
(default)
插入文本或HTML。