UploadField max file size validation has no effect for large files

[kinglozzer]

Tested with 1.3.3

/** @var Upload_Validator $validator */
$validator = $uploadField->getValidator();
$validator->setAllowedMaxFileSize('1m');

I’d expect this to prevent the upload, but it actually allows the file to be uploaded before validation is run server-side. That, while a little frustrating for CMS users who have slow connections, does work for smaller files, however...

If post_max_size is set to 10mb, the field is set to 1mb, and the user attempts to upload a 40mb file, they receive a useless error message: a 400 error code and There has been an error (or other webserver error message). Error logs show:

PHP Warning: POST Content-Length of 40500753 bytes exceeds the limit of 10485760 bytes in Unknown on line 0, referer:

Attempting to upload files via AssetAdmin does produce error messages prior to uploading, so client-side validation is obviously possible somehow.

Expected results

• If the set UploadField max file size is exceeded, there is client-side validation alerting the user
• The file is prevented from being uploaded/does not reach the server

PRs

• ADD Upload_Validator::getLargestAllowedMaxFileSize method silverstripe-assets#341
• FEATURE UploadField dropzone validates filesize before uploading #1017

[dnsl48]

Done. Tested in IE11 and Chromium