The plupload uploader uses only one max_file_size variable in it's config. It's the one defined in ACP > Borad Configuration  > Attachment settings > Maximum file size.

In the same Attachment settings page, there's another field called `Maximum file size messaging`, that's expected to limit the size of the files attached to PMs.

When trying to upload a file in a PM, if the file gets over the `Maximum file size` setting, but it is below the `Maximum file size messaging` setting, an error alert will be shown.
Same, if the `Maximum file size` is above, and `Maximum file size messaging` is below the file size, JS won't show the alert to notify the user that the file is too big.

Steps to reproduce:

1. Make sure JS is on in your browser.
2. Go to settings, and make sure that General > Private message settings > `Allow attachments in private messages` is set to "Yes".
3. Go to settings, and set: General > Attachment settings > `Maximum file size` to 5 Bytes.
4. Go to settings, and set: General > Attachment settings > `Maximum file size messaging` to 512 MegaBytes.
5. Go to PM, try writing a new message, go to attachments, and attach a file.