[PLT-5445] Mobile view: Set focus back to message box after uploading a file (#5360)

webapp/components/file_upload.jsx

@@ -126,6 +126,8 @@ class FileUpload extends React.Component {
 
             Utils.clearFileInput(e.target);
         }
+
+        this.props.onFileUploadChange();
     }
 
     handleDrop(e) {
@@ -190,6 +192,8 @@ class FileUpload extends React.Component {
                 self.handleDrop(e);
             }
         });
+
+        this.props.onFileUploadChange();
     }
 
     componentWillUnmount() {
@@ -289,6 +293,8 @@ class FileUpload extends React.Component {
                 this.props.onUploadStart([clientId], channelId);
             }
         }
+
+        this.props.onFileUploadChange();
     }
 
     keyUpload(e) {
@@ -369,6 +375,7 @@ FileUpload.propTypes = {
     onClick: React.PropTypes.func,
     onFileUpload: React.PropTypes.func,
     onUploadStart: React.PropTypes.func,
+    onFileUploadChange: React.PropTypes.func,
     onTextDrop: React.PropTypes.func,
     channelId: React.PropTypes.string,
     postType: React.PropTypes.string