diff --git a/.php-cs-fixer.dist.php b/.php-cs-fixer.dist.php
index 2f829560..d68945fd 100644
--- a/.php-cs-fixer.dist.php
+++ b/.php-cs-fixer.dist.php
@@ -21,7 +21,7 @@
         'no_break_comment' => false,
         'no_extra_blank_lines' => true,
         'no_spaces_around_offset' => true,
-        'no_trailing_comma_in_singleline' => true,
+        'no_trailing_comma_in_singleline_array' => true,
         'no_unused_imports' => true,
         'no_useless_else' => true,
         'no_useless_return' => true,
diff --git a/spec/Gaufrette/FilesystemSpec.php b/spec/Gaufrette/FilesystemSpec.php
index 6924bad5..f40b13b0 100644
--- a/spec/Gaufrette/FilesystemSpec.php
+++ b/spec/Gaufrette/FilesystemSpec.php
@@ -322,7 +322,7 @@ function it_delegates_checksum_calculation_to_adapter_when_adapter_is_checksum_c
         $extendedAdapter->read('filename')->shouldNotBeCalled();
         $extendedAdapter->checksum('filename')->shouldBeCalled()->willReturn(12);
 
-        $this->checksum('filename')->shouldReturn("12");
+        $this->checksum('filename')->shouldReturn('12');
     }
 
     function it_delegates_mime_type_resolution_to_adapter_when_adapter_is_mime_type_provider(ExtendedAdapter $extendedAdapter)
diff --git a/src/Gaufrette/Adapter/AsyncAwsS3.php b/src/Gaufrette/Adapter/AsyncAwsS3.php
index 972cdf38..a11b7cd9 100644
--- a/src/Gaufrette/Adapter/AsyncAwsS3.php
+++ b/src/Gaufrette/Adapter/AsyncAwsS3.php
@@ -176,11 +176,11 @@ public function size(string $key): int
         return (int) $result->getContentLength();
     }
 
-    public function mimeType(string $key): string
+    public function mimeType(string $key): string|bool
     {
         $result = $this->service->headObject($this->getOptions($key));
 
-        return $result->getContentType();
+        return $result->getContentType() ?: false;
     }
 
     /**
diff --git a/src/Gaufrette/Adapter/AwsS3.php b/src/Gaufrette/Adapter/AwsS3.php
index ace91dc4..9e9141de 100644
--- a/src/Gaufrette/Adapter/AwsS3.php
+++ b/src/Gaufrette/Adapter/AwsS3.php
@@ -173,7 +173,7 @@ public function size(string $key): int
         }
     }
 
-    public function mimeType(string $key): string|bool
+    public function mimeType(string $key): bool|string
     {
         try {
             $result = $this->service->headObject($this->getOptions($key));
diff --git a/src/Gaufrette/Adapter/AzureBlobStorage.php b/src/Gaufrette/Adapter/AzureBlobStorage.php
index f94d03b2..5352e5bc 100644
--- a/src/Gaufrette/Adapter/AzureBlobStorage.php
+++ b/src/Gaufrette/Adapter/AzureBlobStorage.php
@@ -286,7 +286,7 @@ public function size(string $key): int
     /**
      * {@inheritdoc}
      */
-    public function mimeType(string $key): string
+    public function mimeType(string $key): string|bool
     {
         $this->init();
         list($containerName, $key) = $this->tokenizeKey($key);
diff --git a/src/Gaufrette/Adapter/InMemory.php b/src/Gaufrette/Adapter/InMemory.php
index 7f76b388..f87d6892 100644
--- a/src/Gaufrette/Adapter/InMemory.php
+++ b/src/Gaufrette/Adapter/InMemory.php
@@ -120,7 +120,7 @@ public function isDirectory(string $path): bool
         return false;
     }
 
-    public function mimeType(string $key): string
+    public function mimeType(string $key): bool|string
     {
         $fileInfo = new \finfo(FILEINFO_MIME_TYPE);
 
diff --git a/src/Gaufrette/Adapter/Local.php b/src/Gaufrette/Adapter/Local.php
index e19af1a1..e56da5f0 100644
--- a/src/Gaufrette/Adapter/Local.php
+++ b/src/Gaufrette/Adapter/Local.php
@@ -183,11 +183,12 @@ public function size(string $key): int
     }
 
     /**
+     * {@inheritdoc}
      * @throws \OutOfBoundsException     If the computed path is out of the directory
      * @throws \InvalidArgumentException if the directory already exists
      * @throws \RuntimeException         if the directory could not be created
      */
-    public function mimeType(string $key): string
+    public function mimeType(string $key): bool|string
     {
         $fileInfo = new \finfo(FILEINFO_MIME_TYPE);
 
diff --git a/src/Gaufrette/Adapter/MimeTypeProvider.php b/src/Gaufrette/Adapter/MimeTypeProvider.php
index 53617bea..863e9a75 100644
--- a/src/Gaufrette/Adapter/MimeTypeProvider.php
+++ b/src/Gaufrette/Adapter/MimeTypeProvider.php
@@ -10,7 +10,7 @@
 interface MimeTypeProvider
 {
     /**
-     * Returns the mime type of the specified key.
+     * @return false|string the mime type of the specified key.
      */
-    public function mimeType(string $key): string;
+    public function mimeType(string $key): string|bool;
 }
diff --git a/src/Gaufrette/FilesystemInterface.php b/src/Gaufrette/FilesystemInterface.php
index 9a686ab8..57d16dd7 100644
--- a/src/Gaufrette/FilesystemInterface.php
+++ b/src/Gaufrette/FilesystemInterface.php
@@ -167,7 +167,7 @@ public function createFile($key);
      *
      * @param string $key
      *
-     * @return string
+     * @return string|false
      *
      * @throws \InvalidArgumentException If $key is invalid
      */