Allow multiple file uploads at once

The file input now allows selecting multiple files, and the event
handler will now upload all of the selected files to the local storage.
Support for image files and view its contents

Image files can be viewed as an image now. Before it could only be
viewed as binary code in the Grace editor. In order to achieve this, a
new section with id="image-view" has been added to the HTML which
contains the image display. A new function called isImage is in files.js
to identify which files are images based on their name.
Support for audio files and play its contents

Audio files can now be played. Before it could only be viewed as binary
code. There are three supported file formats for audio files: mp3, ogg
and wav. Users can play and pause the audio, and adjust volume.
Ask to rename a file if it already exists

The user is required to rename the uploaded or created file if it
already exists. The rule is suitable for all file formats now. Before it
only worked for Grace files. If the new file name has no extension, then
the old extension will be added.