The message you get when you post a file that is too much big is not perfectly clear for end users.

I suggest:
- adding the size detected by filesize() in the first case
- remove references to character escaping
- use bytes instead of characters
- switch to kB/MB

Btw, why do files must be bigger than 20 chars?

frontend/php/include/trackers/general.php:

# Check file size:
# - first with filesize(), that can result appropriate result for big files
# unlike strlen. If it is too big, we stop here.
# - then with strlen after the call the addslashes(): the check must be
# made on the final size for import in the database.
#
if (filesize($input_file) > 512000) {
fb(_("File not attached: must be > 20 chars and < 512000 chars in length, after escaping characters as required."), 1);
return false;
}
$data = addslashes(fread($data, filesize($input_file)));
if ((strlen($data) < 20) || (strlen($data) > 512000))
{
fb(sprintf(_("File not attached: must be > 20 chars and < 512000 chars in length. After escaping characters as required, it sized %s."), strlen($data)), 1);
return false;
}