Add Attachment structure (#1731)

* Add Attachment structure

* Fix linter issues + @private

* Fixed array sends, also added embed sends

* fixed proving path to attachment

* fixed incorrect name assumption from path

* linting fix

* ;)

* im really good at this

* changes as requested by gus

and computer from #1459

* am a dum

* update webhook#send

* readonly addition to getters

* i... uh... oops

* farming deez commits

* fix webhook split

* removed some ugly

* removed .every checks

src/errors/Messages.js

@@ -63,7 +63,7 @@ const Messages = {
   UDP_CONNECTION_EXISTS: 'There is already an existing UDP connection.',
 
   REQ_BODY_TYPE: 'The response body isn\'t a Buffer.',
-  REQ_RESOURCE_TYPE: 'The resource must be a string or Buffer.',
+  REQ_RESOURCE_TYPE: 'The resource must be a string, Buffer or a valid file stream.',
 
   IMAGE_FORMAT: format => `Invalid image format: ${format}`,
   IMAGE_SIZE: size => `Invalid image size: ${size}`,