html: firmware selection: limit extension only to .bin

This makes it quicker to select firmware image from the output directory.

You can still select at File type "All Files" if the extention is diffrent.

html/update.html

@@ -11,7 +11,7 @@
     <h1>Firmware Update</h1>
     <form enctype="multipart/form-data" action="/upload" method="POST">
     <input type="hidden" name="MAX_FILE_SIZE" value="1000000" />
-      Choose a firmware update file to upload: <input name="uploadedfile" type="file" /><br />
+      Choose a firmware update file to upload: <input name="uploadedfile" type="file" accept=".bin" /><br />
     <input type="submit" value="Upload File" />
     </form>
     <script src="/navigation.js"></script>