you need to edit your upload.php. use the str_replace function to replace spaces with underscores.

generally like this

<?php
$str = ' This is a test ';
$count = 1;
while($count)
$str = str_replace(' ','', $str, $count);
echo $str;
?>

But maybe preg_replace is the better choice to clean everything bad from the filename

$fname="Filen'ame 1\/23;".'"'."la\l[a]*(/.jpg";
$replace="_";
$pattern="/[^a-zA-Z0-9\.]/";
$fname=preg_replace($pattern,$replace,$fname);
$fname=str_replace($replace,'',$fname);
echo '<br>'.$fname;


edit uploadify.php

$var =$_FILES['Filedata']['name'];
$name= ereg_replace(" ","_",$var);
$targetFile = str_replace('//','/',$targetPath) . $name;

like I posted above , with preg_replace

This is my final php, which cleans the filename from spaces and other problematic chars,
randomizes the filename afterwards and send it back to the backend:

<?php
if (!empty($_FILES)) {
$replace="_";
$newfile= $_FILES['Filedata']['name'] ;
$ext = substr($newfile,-4); //comment out to not ramdomize the filename
$pre = mb_split($ext,$newfile); //comment out to not ramdomize the filename
$randoms = base_convert(mt_rand(0x1679616, 0x39AA3FF), 10, 36); //comment out to not ramdomize the filename
$newfile = $pre[0].$randoms.$ext; //comment out to not ramdomize the filename
$pattern="/[^a-zA-Z0-9\.]/";
$newfile = preg_replace($pattern,$replace,$newfile);
$tempFile = $_FILES['Filedata']['tmp_name'];
$targetPath = $_SERVER['DOCUMENT_ROOT'] . $_REQUEST['folder'] . '/';
$targetFile = str_replace('//','/',$targetPath) . $newfile;
move_uploaded_file($tempFile,$targetFile);
chmod($targetFile,0777);
echo $newfile; // Required to trigger onComplete function on Mac OSX (same for linux and windows as it seems)
//and to send back the new filename to the 'response' variable in uploadify
}
else { // Required to trigger onComplete function on Mac OSX
echo 'ERROR!';
}
?>

Actually I don't use the check script. In my case I have to upload hundreds of images and therefore I need to give the user the possibility to upload files, without worrying about the file name as he could want to upload a file named logo.gif for different datasets maybe ten times.
That's why I added a random part to the filename. The $randoms very likely never gives the same result twice.
I think you can't alter the check.php to follow the same rules, as the $randoms will be different even for the same finename, though

I would be possible if you comment out the ramdomising part and only use the 'cleaning' part.
Then the output is predictible.
I haven't tested that, but it should be something like this:

$fileArray = array();
$replace="_";
$pattern="/[^a-zA-Z0-9\.]/";
foreach ($_POST as $key => $value) {
if ($key != 'folder') {
$value= preg_replace($pattern,$replace,$value);
if (file_exists($_SERVER['DOCUMENT_ROOT'] . $_POST['folder'] . '/' . $value)) {
$fileArray[$key] = $value;
}
}
}
echo json_encode($fileArray);

This is just a quick guess, try if it works.
And comment out the lines in upload.php which are indicated.