Hints accepted types in OS in file picker

Now that we have improved support for restricted file types in the file picker, there's no reason not to add hints to the browse button so the OS will only let you select the types that would be accepted.

For reference: http://caniuse.com/input-file-accept/embed But we already have server side checks for this, so that's not an issue.

Example:

<input type="file" accept=".xls,.xlsx" />