Remove all uses of file_get_mimetype

Thanks @chintan.vyas!

+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -446,7 +446,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       }
 
       if (empty($values['default_favicon']) && !empty($values['favicon_path'])) {
-        $values['favicon_mimetype'] = file_get_mimetype($values['favicon_path']);
+        $values['favicon_mimetype'] = \Drupal::service('file.mime_type.guesser')->guess($values['favicon_path']);
       }
     }

This should be injected instead of calling \Drupal::service(). Have a look at the patch in the parent issue #2392559-37: Remove all uses of file_stream_wrapper_get_* and file_get_stream_wrappers where that's still in.

Just a couple of doc issues

+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -38,6 +39,12 @@ class ThemeSettingsForm extends ConfigFormBase {
+   * The MIME type guesser.
+   * @var \Symfony\Component\HttpFoundation\File\MimeType\MimeTypeGuesserInterface

There should be an empty comment line between the description and the @var.

+++ b/core/modules/system/src/Form/ThemeSettingsForm.php
@@ -49,16 +56,19 @@ class ThemeSettingsForm extends ConfigFormBase {
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface module_handler

Missing $ in front of the $module_handler variable.

Can you also post an interdiff in a new patch?

Thanks

Patch in #5 removes all usages of deprecated file_get_mimetype, and fixes two minor doc issues.

RTBC

Created #2417071: Remove file_get_mimetype for function removel, added beta evaluation