Joomla file Form Field Type
It provides an input field for files. When using the file input type, you should always add the attribute enctype="multipart/form-data" to your form tag. Otherwise, the uploaded files will not be attached correctly.
Example XML field definition:
<field name="myfile"
type="file"
label="File"
description="Choose an image with maximum 100KB"
accept="image/*" />
Additional Parameters
accept: It tells the browser what MIME types your form will allow to be uploaded.
multiple: It allows you to upload multiple files at once when set to multiple="true".