Add ability to include embed with upload_file (#16)

src/discordcr/rest.cr

@@ -433,7 +433,7 @@ module Discord
     # [API docs for this method](https://discord.com/developers/docs/resources/channel#create-message)
     # (same as `#create_message` -- this method implements form data bodies
     # while `#create_message` implements JSON bodies)
-    def upload_file(channel_id : UInt64 | Snowflake, content : String?, file : IO, filename : String? = nil, spoiler : Bool = false)
+    def upload_file(channel_id : UInt64 | Snowflake, content : String?, file : IO, filename : String? = nil, embed : Embed? = nil, spoiler : Bool = false)
       io = IO::Memory.new
 
       unless filename
@@ -449,8 +449,14 @@ module Discord
       end
 
       builder = HTTP::FormData::Builder.new(io)
-      builder.field("content", content) if content
       builder.file("file", file, HTTP::FormData::FileMetadata.new(filename: filename))
+      if content || embed
+        json = encode_tuple(
+          content: content,
+          embed: embed
+        )
+        builder.field("payload_json", json)
+      end
       builder.finish
 
       response = request(