Added file uploading + drag'n'drop

src/app/chat/elements/message-box/message-box.html

@@ -1,5 +1,5 @@
 <div id="dragOverlay" tuiGroup orientation="vertical" [class.show]="isDraggingOverWindow">
-  <div class="method">
+  <div class="method" (drop)="onDrop($event)" (dragover)="onDragOver($event)">
     <div class="icon-holder">
       <tui-icon icon="@tui.file-up"/>
     </div>
@@ -33,7 +33,7 @@
       ></textarea>
     </tui-textfield>
 
-    <button tuiButton iconStart="@tui.send" (click)="sendMessageWithCaption(caption)"></button>
+    <button tuiButton iconStart="@tui.send" (click)="sendMessageWithCaption()"></button>
   </div>
 </ng-template>