Update antimalware.py to be more consistent with other information messages & improve code a slight bit

[Denayder]

Hey! I looked at the antimalware.py and felt like I could improve a bit, and I hope I've accomplished that. I look forward to your review of this pull request! 😁

Here's a list of differences that has been made:

• Messages are now only checked if they contain 1 or more attachments.
• Warning message is now in an embed, but the mention is still text so the user still gets pinged. This is more consistent with other reminders (such as this one: https://prnt.sc/q1t6g7)
• Less if statements and variables
• Removed the try/except block: I don't believe deleting a message from on_message can ever return NotFound. If I'm wrong, I'm more than happy to add that back in.
• If a user sends an illegal file, a list of allowed file types are displayed in the message as well, so they know what they can use.

I think this is an overall improvement to the message and code itself, and I hope you guys agree 😁

[Akarys42]

Looks pretty good, just two small things

[Denayder]

All requested tweaks have been made :)

[Akarys42]

Have you created the default constant in the config-default.yml file? My bad it is already in it

[Akarys42]

👌

[avayert]

LGTM

[lemonsaurus]

Good refactor.