better visual style of broken images

src/widgets/chat-message.jsx

@@ -181,6 +181,7 @@ function draftyFormatter(style, data, values, key) {
           attr.style = { width: dim.dstWidth + 'px', height: dim.dstHeight + 'px' };
           if (!Drafty.isProcessing(data)) {
             attr.src = this.props.tinode.authorizeURL(sanitizeImageUrl(attr.src));
+            attr.alt = data.name;
             if (attr.src) {
               attr.onClick = this.handleImagePreview;
               attr.className += ' image-clickable';

src/widgets/image-preview.jsx

@@ -67,7 +67,7 @@ export default class ImagePreview extends React.PureComponent {
           <a href="#" onClick={(e) => {e.preventDefault(); this.props.onClose();}}><i className="material-icons gray">close</i></a>
         </div>
         <div id="image-preview-container" ref={(node) => this.assignWidth(node)}>
-          <img src={this.props.content.url} style={size} />
+          <img src={this.props.content.url} style={size} className="image-preview" alt={this.props.content.filename} />
         </div>
         {this.props.onSendMessage ?
           <SendMessage