OptionalacceptWhat the open-file picker should accept for this format — MIME types, .ext
tokens, or both, exactly as an <input type="file" accept> takes them.
Defaults to ['.<ext>'], which is enough for a format the OS has no MIME type for.
Extension without the leading dot, e.g. 'docx'. Matched case-insensitively
against the end of the file name.
One extra openable format declared by the host.