bootstrap-vue/es/components/form-file/README.md

BootstrapVue provides one of the most comprehensive implementations of Bootstrap 4 components and grid system for Vue.js and with extensive and automated WAI-ARIA accessibility markup.

# Form File Input

> Customized, cross-browser consistant, file input control that supports single file,
multiple files, and directory upload (for browsers that support directory mode)

```html
<template>
<div>
  <!-- Styled -->
  <b-form-file v-model="file" :state="Boolean(file)" placeholder="Choose a file..."></b-form-file>
  <div class="mt-3">Selected file: {{file && file.name}}</div>

  <!-- Plain mode -->
  <b-form-file v-model="file2" class="mt-3" plain></b-form-file>
  <div class="mt-3">Selected file: {{file2 && file2.name}}</div>
</div>
</template>

<script>
export default {
  data () {
    return {
      file: null,
      file2: null
    }
  }
}
</script>

<!-- form-file.vue -->
```

For cross browser consistency, Form file defaults to the Bootstrap custom file
input to replace the browser defaults. They’re built on top of semantic and accessible
markup, so it is a solid replacement for the default file input.

## Single file (default)
On single file mode, when no file is selected or user cancels Browse dialog, `v-model` is `null`
indicating no file selected. When a file is selected the return value will be a javascript
[`File`](https://developer.mozilla.org/en/docs/Web/API/File) object instance.

## Multiple files
Multiple file uploading is supported by adding `multiple` prop to component.
In this case `v-model` is *always* an `Array`. When no files are selected, an empty array
will be returned. When a file or files are selected the return value will be an array of
javascript [`File`](https://developer.mozilla.org/en/docs/Web/API/File) object instances.

## Directory mode
By adding `directory` prop, the user can select directories instead of files.
When a directory is selected, the directory and its entire hierarchy of contents are included in the set of selected items.
The selected file system entries can be obtained using the `webkitEntries` property.

**CAUTION** This is a non standard feature while being supported by latest Firefox and Chrome versions, and should not
be relied for production.
[Read more on MDN](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/webkitdirectory)

## Drag and Drop
Drop mode is enabled by default. it can disabled by setting the `no-drop` prop.

## Limiting to certain file types
You can limit the file types by setting the `accept` prop to a string containing the
allowed file type(s). To specify more than one type, separate the values with a comma.

```html
<!-- Accept all image formats by IANA media type wildcard-->
<b-form-file accept="image/*"></b-form-file>

<!-- Accept specific image formats by IANA type -->
<b-form-file accept="image/jpeg, image/png, image/gif"></b-form-file>

<!-- Accept specific image formats by extension -->
<b-form-file accept=".jpg, .png, .gif"></b-form-file>
```

To accept any file type, leave `accept` as null (default). You can mix and match IANA
media types and extensions.

Refer to [IANA Media Types](http://www.iana.org/assignments/media-types/) for a complete
list of standard media types.

**Note:** Not all browsers support or respect the `accept` attribute on file inputs.

## Customize browse label
If you want to globally change `Browse` label, you can add something like this to your global stylesheets.
Also it is advised to use [:lang()](https://developer.mozilla.org/en-US/docs/Web/CSS/:lang) for multi-language sites.

```css
.custom-file-input:lang(en)~.custom-file-label::after {
    content: "Browse";
}
```

## Non custom file input
You can have `<b-form-file>` render a browser native file input by setting the `plain` prop.

## Contextual state feedback
Bootstrap includes validation styles for `valid` and `invalid` states
on most form controls.

Generally speaking, you’ll want to use a particular state for specific types of feedback:
- `'invalid'` is great for when there’s a blocking or required field. A user must fill in
this field properly to submit the form.
- `'valid'` is ideal for situations when you have per-field validation throughout a form
and want to encourage a user through the rest of the fields.
- `null` Displayes no validation state

To apply one of the contextual state icons on `<b-form-file`, set the `state` prop
to `'invalid'` (or `false`), `'valid'` ( or `true`), or `null`.

## Accessibility
When using the custom version of  `<b-form-file>` input which hides the original input, it is
**highly recommended** that you supply a document unique ID string via the `id` prop. This will
automatically render the extra ARIA atributes required to improve usability for persons using
assitive technologies.

## Clearing the file selection
Because of limitations in the value binding with `<input type="file">` elements, `v-model`
for `<b-form-file>` is unidirectional, and cannot be used to set or clear the file(s) selection.
To get around this limitation, `<b-form-file>` provides a `reset()` method that can be
called to clear the file input.

To take advantage of the `reset()` method, you will need to obtain a reference
to the `<b-form-file>` component:

```html
<div id="#app">
    <b-form-file v-model="file" ref="fileinput"></b-formfile>
    <b-button @click="clearFiles">Reset</b-button>
</div>
```

```js
window.app = new Vue({
  el: '#app',
  data () {
    return {
      file: null
    }
  },
  methods: {
    clearFiles () {
      this.$refs.fileinput.reset();
    }
  }
})
```

## Component alias

`<b-form-file>` can be used via the shorter component alians `<b-file>`.

## Component Reference