Upload multipart/form-data from memory in Python with aiohttp

I am trying to work with Discord.py to upload an attachment to Ballchasing's API. Here are the relevant API sections:

https://discordpy.readthedocs.io/en/latest/api.html#discord.Attachment.read

https://ballchasing.com/doc/api#upload-upload-post

The example in the docs suggest using requests, but I've read over and over again that this isn't best practice for a Discord bot because you want asynchronous code to avoid anything that could block the execution of your script.

Here is what I have:

@commands.Cog.listener()        
async def on_message(self, message):
    headers = {'Authorization':self.upload_key_bc}
    for attachment in message.attachments:
        file = io.BytesIO(await attachment.read())
        action = {'file': ('replay.replay', file.getvalue())}
        async with aiohttp.ClientSession() as session:
            async with session.post(self.api_upload_bc, headers=headers, data=action) as response:
                print(response.status)
                print(await response.text())

I'm getting this response:

failed to get multipart form: request Content-Type isn't multipart/form-data

I tried forcing the Content-Type header to multiparth/form-data and I get a different error:

failed to get multipart form: no multipart boundary param in Content-Type

I think the way I'm sending the data is the problem. What am I missing?

from Upload multipart/form-data from memory in Python with aiohttp