diff -u b/core/modules/file/src/Entity/File.php b/core/modules/file/src/Entity/File.php
--- b/core/modules/file/src/Entity/File.php
+++ b/core/modules/file/src/Entity/File.php
@@ -254,7 +254,7 @@
 
     $fields['filemime'] = BaseFieldDefinition::create('string')
       ->setLabel(t('File MIME type'))
-      ->setOption('is_numeric', TRUE)
+      ->setOption('is_alphanumeric', TRUE)
       ->setDescription(t("The file's MIME type."));
 
     $fields['filesize'] = BaseFieldDefinition::create('integer')