Issue #2389455 by hussainweb, AjitS, Mile23, subhojit777, tibbsa, Ayesh: Clean-up system module test members - ensure property definition and use of camelCase naming convention

Issue #2457887 by prateekMehta, stefan.r, rpayanm, alexpott: Use Utility\SafeMarkup class instead of Utility\String for placeholder(), checkPlain(),format() functions

Issue #2050759 by tim.plunkett, fietserwin: Move drupal_chmod and other code in file.inc to a Drupal\Core\File\FileSystem class

Revert "Issue #2388749 by alexpott, adci_contributor, ParisLiakos: Register symfony's mime guessers if they are supported"

This reverts commit 4dae1d99.

Issue #2388749 by alexpott, adci_contributor, ParisLiakos: Register symfony's mime guessers if they are supported

Issue #2382195 by tibbsa, subhojit777, tadityar, cilefen, Mile23: Clean-up simpletest module test members - ensure property definition and use of camelCase naming convention

Issue #2395395 by alexpott, effulgentsia, Gábor Hojtsy: TestBase lacks a config method to be used consistently in tests

Issue #2353357 by Berdir: hook_stream_wrappers_alter() is broken since modules are not loaded on demand, also change to an event since it's the last hook that forces this during bootstrap

Issue #2170235 by Berdir, penyaskito, larowlan, alexpott, sushyl: file_private_path should be in $settings, like file_public_path

Issue #2347999 by Mile23, daffie, cs_shadow, cosmicdreams: DrupalUnitTestBase is deprecated, replace with KernelTestBase

Issue #2276203 by webflo, lauriii, nlisgo, kallehauge, Snipon: Fixed CSS Aggregation breaks URLs with Query String.

Issue #2028109 by tim.plunkett, almaudoh, twistor, larowlan, slashrsm, Berdir, Arla, neclimdul, Cottser: Convert hook_stream_wrappers() to tagged services.

Issue #2112247 by sihv, dgroene, aalamaki, Dennis Walgaard, mErilainen: Fixed Valid file extensions in file names are not properly enforced when uploading files.

Issue #2284103 by alexpott, fabpot, damiankloip, Xano, Xen, Berdir: Fixed Remove the request from the container.

Issue #697760 by sun: Replace getInfo() in tests with native phpDoc + annotations (following PHPUnit).

Issue #1921558 by ParisLiakos, xjm, Dave Reid: Convert file_get_mimetype() to use Symfony MimeTypeGuessers.