upload_file actually does require a file name

[greenbigfrog]

discordcr/src/discordcr/rest.cr

Lines 430 to 431 in df22ea8

	# the file's filename will be used instead. If it is an `IO` without
	# filename information, Discord will generate a placeholder filename.

Not sure where the issue lies, but trying to send a IO without a filename, resulted in discord dropping the upload altogether.
Further investigation required.