Fix showing thumb when sending document (#389)

[Abstract-X]

Description

import asyncio
from aiogram import Bot
from aiogram.types import InputFile

BOT_TOKEN = "..."
CHAT_ID = ...

async def main():
    bot = Bot(token=BOT_TOKEN)
    await bot.send_document(chat_id=CHAT_ID,
                            document=InputFile("document.zip"),
                            thumb=InputFile("thumb.jpg"))
    await bot.close()

asyncio.run(main())

Fixes #389

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes