After reading through the comments on this post, I came up with the following syntax for the accept attribute:
Images
<input type="file" accept="image/jpeg, image/png, image/gif, .jpeg, .png, .gif">
Audio
<input type="file" accept="audio/mpeg, audio/x-wav, .mp3, .wav">
This works perfectly on desktop browsers, but does not appear to filter files at all on iOS or Android.
Are there any cross-browser solutions available?