Steps to reproduce the issue

Install Joomla 4 on a server that requires the use of the FTP Layer (see #31857)

Navigate System -> Site Templates -> Cassiopeia Details and Files -> index.php

Click Save

Expected result

File is saved.

Actual result

FTP logs a tmp file upload:

Thu Apr 22 16:58:25 2021 [pid 75051] CONNECT: Client "::ffff:127.0.0.1"
Thu Apr 22 16:58:25 2021 [pid 75050] [www-ftp] OK LOGIN: Client "::ffff:127.0.0.1"
Thu Apr 22 16:58:25 2021 [pid 75052] [www-ftp] OK UPLOAD: Client "::ffff:127.0.0.1", "/tmp/bb2fa7e9933b1b1096cd4ee9a625476e", 0.00Kbyte/sec
Thu Apr 22 16:58:25 2021 [pid 75052] [www-ftp] OK DELETE: Client "::ffff:127.0.0.1", "/tmp/bb2fa7e9933b1b1096cd4ee9a625476e"

The ACTUAL index.php file is NOT written/updated

Then there is a yellow warning (ignore the red warning - that is from #33238)

NOTE the permissions are read with Path::getPermissions($filePath) which DOESNT take into account the FTP Layer

NOTE that saving is also failing because of another is_writable call

System information (as much as possible)

Additional comments