I was testing your new Discord webhook (thank you!). I have also been using Rustcord to help with this.
I was pasting an image in-game and it worked fine but there was no output to the webhook channel I specified or in Rustcord (error in RCON below).
I then realized I wasn't supplying http or https with /sil. After I entered http or https, then it worked as expected: pasted in-game and then showed up in my Discord channels.
I suggest you may want to add logic to require http or https and outputting error message to user in-game. Thanks!
(00:26:15) | [Discord Extension] An error occured whilst submitting a request to https://discordapp.com/api/channels/691438798441152552/messages (code BadRequest): {"embed": ["image"]}