Client-side image verification not detecting wrong file type

[wlabarron]

When submitting an image of a type which the system doesn't support, the server-side verification will reject it, but the client-side will not.

[wlabarron]

The validation in HTML is an accept="image/png,image/jpeg" attribute on the image upload form input and a file size check in JS, followed by a similar server-side check in PHP (but mostly based on extensions rather than MIME types). I don't have an example image to hand which caused this problem, so need to put this on hold til it happens again and I can find an image which triggers the problem.