Couple of suggestions. 1. It would make more sense to have Textbox at the bottom instead of top. When you enter something, you see your message right above your textbox.

2. Instead of having separate area for dropping files, why not allow users to drop files in the same textbox where you type your message.

3. Bubbles doesn't look nice. I mean, it looks good as a demo but not when you use the product. It is sort of a distraction. I would rather like to see zebra stripes(may be #ffffff and #f1f1f1).

4. I like to see list of users on the sidebar instead of on chat area.