On Windows 8, Chrome and firefox, on my laptop I can upload doc, docx and pdf files no problem. But on the PCs next to me, also windows 8, it fails to upload doc and docx in any browser, saying:
"There was an error with your submission
This kind of file is not allowed. Please, try again selecting other file.:
on my laptop where everything works with chrome and firefox, it rarely works in internet explorer (8) - but intermittently it works. 95% of the time it just takes me to /wp-admin/admin-ajax.php which dies with a browser-based error message "This page can't be displayed" and a button for fixing connection problems.
here's an example page - the form is at the bottom of the page and you can see the error messages when you try uploading: http://seojobcentre.co.uk/london/2013/07/16/michael-page-marketing-head-of-online/
note that there's absolutely no difference between my pcs and laptops except that one's a laptop. so it really seems weird that it workson laptop but not on pcs. the i.e. bugs i half expect, but not the alternate pc bugs