Allow input type file for NcActionInput

I want to make an app that does some preprocessing on files before uploading to Nextcloud. For this, I need a file input. However, the NcActionInput does not support type=file :

<NcActionInput
		type="file"
		accept=".csv"
		@submit="$emit('upload-file', $event)">
		<template #icon>
			<UpIcon />
		</template>
</NcActionInput>

<script>
import NcActionInput from '@nextcloud/vue/dist/Components/NcActionInput.js'
import UpIcon from 'vue-material-design-icons/ArrowUpThin.vue'
</script>

I would be happy to PR if people think it is a good idea

Start with the NcActionInput component and compare its current behavior with the component documentation linked in the issue. Trace how the type and accept values reach the underlying input and how submit emits its event. Done means a file input can accept the requested file types and emit the selected file through upload-file.