Allow sending files list smaller than 2 elements in Messageable.send
Allow sending files list smaller than 2 elements in Messageable.send

The previous restriction was unwarranted.

imayhaveborkedit/discord.py,mgardne8/discord.py,rapptz/discord.py,Harmon758/discord.py,Rapptz/discord.py,Harmon758/discord.py,khazhyk/discord.py