Add sendFile function (#562)

* sendFile

* Add default value to filename

* eslint

* (╯°□°）╯︵ ┻━┻

src/structures/interface/TextBasedChannel.js

@@ -68,7 +68,24 @@ class TextBasedChannel {
   sendTTSMessage(content, options = {}) {
     return this.client.rest.methods.sendMessage(this, content, true, options.nonce);
   }
-
+  /**
+   * Send a file to this channel
+   * @param {FileResolvable} attachment The file to send
+   * @param {String} [fileName="file.jpg"] The name and extension of the file
+   * @returns {Promise<Message>}
+   */
+  sendFile(attachment, fileName = 'file.jpg') {
+    return new Promise((resolve, reject) => {
+      this.client.resolver.resolveFile(attachment)
+      .then(file => {
+        this.client.rest.methods.sendMessage(this, undefined, false, undefined, {
+          file,
+          name: fileName,
+        }).then(resolve).catch(reject);
+      })
+      .catch(reject);
+    });
+  }
   /**
    * The parameters to pass in when requesting previous messages from a channel. `around`, `before` and
    * `after` are mutually exclusive. All the parameters are optional.
@@ -173,7 +190,7 @@ function applyProp(structure, prop) {
 }
 
 exports.applyToClass = (structure, full = false) => {
-  const props = ['sendMessage', 'sendTTSMessage'];
+  const props = ['sendMessage', 'sendTTSMessage', 'sendFile'];
   if (full) {
     props.push('_cacheMessage');
     props.push('getMessages');