upload_file actually does require a file name

[greenbigfrog]

https://github.com/shardlab/discordcr/blob/df22ea8f9499756949dfa16ed629bd1724b77e05/src/discordcr/rest.cr#L430-L431

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.