The value of the accept attribute is, as per HTML5 LC, a comma-separated list of items, each of which is a specific media type like image/gif, or a notation like image/* that refers to all image types, or a filename extension like .gif. IE 10+ and Chrome support all of these, whereas Firefox does not support the extensions. Thus, the safest way is to use media types and notations like image/*, in this case
<input type="file" name="foo" accept=
"application/msword, application/vnd.ms-excel, application/vnd.ms-powerpoint,
text/plain, application/pdf, image/*">
if I understand the intents correctly. Beware that browsers might not recognize the media type names exactly as specified in the authoritative registry, so some testing is needed.