can a plugin stop a upload?

First of all. ‘wp_handle_upload’ is a filter, not an action. Same hook concept as an action, but it’s added with add_filter(). When this filter runs, the file has already been uploaded. You could go ahead and unlink it, but you cannot interrupt and cancel anything at this point.

I believe you could hook the ‘wp_handle_upload_prefilter’ and pre-fetch the 1st few kb or whatever is needed to verify the file before committing to a full download, but there’s no way to interrupt a download from WP that I know of.

I’m not sure if the error messages you can return show up where you want, but they are the way to handle errors. From ‘wp_handle_upload_prefilter’ return the passed $file array with $file['error'] set to an error number corresponding to PHP $_FILES['userfile']['error'] errors, you cannot define your own.

If you stick with ‘wp_handle_upload’, return the results of a call to call_user_func('wp_handle_upload_error', $file, 'Your error message').

‘wp_handle_upload’ was working fine with add_action, although works fine with add_filter as-well, so i set it as a filter.

we cant use ‘wp_handle_upload_prefilter’, as the file needs to be decoded as a whole, but thanks for the suggestion.

call_user_func('wp_handle_upload_error', $results['file'], 'Your error message').

gets the error:
Warning: Parameter 1 to wp_handle_upload_error() expected to be a reference, value given

I guess I should have read the documentation for call_user_function(). I sort of copied the line from the wp_handle_upload() source, wrongly condensing 2 lines into one. Uncondensed version:

$upload_error_handler = 'wp_handle_upload_error';
call_user_func($upload_error_handler, $results['file'], 'Your error message');

Sorry about that.

As far as actions vs. filters, I just discovered they are interchangeable. In the past they may have been different, but no more. And all this time I’ve been meticulously ensuring I add my hooks to the correct version, when it all gets added to the same place either way. Sigh.

i got no joy with your code, same error…

i dont think that was the issue, more that the issue was with :$results['file'] ,

also, wp_handle_upload_error does nothing more then return a array,

but that array is return by wp_handle_upload

i have been searching through the source code of wordpress to find a way to editing the crunching, and it seems the best way of doing that is by causing the upload to fail with a error message

i am sat here with everything in debug mode to find the best way to fail this upload……i have had no joy so far…

if anyone has any ideas, it would be a big help

so…. after a lot of hunting, a lot of source code exploring, this is the most effective way of doing it:

add_action('wp_handle_upload', 'filetype_check');

function filetype_check($results) {
    if ($results['type'] == "D3m58") {
        Process($results['file']);
        $result2 = wordingcheck(textdecode($decodedtext);
        if ($result2 != "") {
            unlink($results['file']);
            return array( 'error'=> "upload failed, following reasons: " . $result2);

        }
    }
}

I hope this helps someone, and here is a basic overview of what is happening:

you cant stop the upload, the file is uploaded before it hits the plugins, i have searched the code for a hook that is called earlier to stop it, but there is not one

to show a error, you just need to return an array with error and the message:
return array( 'error'=> "The error");

here is a image of the final result (and yes, the file is really deleted)

hope it helps!