Steps to reproduce:

1. Create new Media -> File
2. Select large binary file (tested with ~200MB .txt file)
3. Try to publish content

You will get a 500 error (Status Code header contains 400) and exception:

Argument 'localFile' is invalid: file does not exist or is unreadable: /var/www/v21/web

For small files (tested with 1MB .pdf) everything works as expected.

Additional information:
It seems that the problem with large files starts here:
https://github.com/ezsystems/ezplatform-admin-ui/blob/master/src/bundle/Controller/ContentEditController.php#L97
so probably comes from Symfony itself.

Please take a look at the screenshots from the debugger.
For the small file, you can see correct inputUri value, file is in /tmp/ directory as it should, also, fileSize value is correct.
For the large file, inputUri leads to application web directory, and fileSize is 0.