Add a file extension check on the validator.

[lypwig]

Considering the input:

<input id="file-button" type="file" accept=".png, .bmp" data-ext="accept">

Then, if you select a file with a wrong extension (it's possible even if you fill the accept attribute), this will display the error: This file type is not supported.

You can also set your valid extensions with: data-ext=".png, .bmp", in this way you don't have to add the accept attribute.

Note: I think than add the data-ext attribute without argument is a more purified syntax, but I'm waiting for #118 .