Sending a GIF from gfycat result in attachment to download

## Description of the problem
I'm trying to send a Gif from gfycat to a Group, it results in an attachement in the thread instead of a preview of it...

## Code to reproduce
def send_image(image_path):
client = Client(login, password)
client.sendRemoteFiles([image_path], thread_id=thread_id, thread_type=ThreadType.GROUP)

Note that the same thing happen with sendLocalFiles (by appending .gif or not in the filename)

## Environment information
- Python version : 3.7.4
- `fbchat` version : 1.7.3

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