MC-31708: Magento Admin order creation Downloading Blank.html File

- Fix http response file content types should be determined based on file extension if the native mime type is generic (e.g application/octet-stream)

Author: bubasuma

dev/tests/static/testsuite/Magento/Test/Legacy/_files/copyright/blacklist.php

@@ -10,4 +10,5 @@
     '/setup\/src\/Zend\/Mvc\/Controller\/LazyControllerAbstractFactory\.php/',
     '/app\/code\/(?!Magento)[^\/]*/',
     '#dev/tests/setup-integration/testsuite/Magento/Developer/_files/\S*\.xml$#',
+    '/lib\/internal\/Magento\/Framework\/File\/Test\/Unit\/_files\/blank.html$/'
 ];

lib/internal/Magento/Framework/File/Mime.php

@@ -78,6 +78,18 @@ class Mime
         'svg' => 'image/svg+xml',
     ];
 
+    /**
+     * List of generic MIME types
+     *
+     * The file mime type should be detected by the file's extension if the native mime type is one of the listed below.
+     *
+     * @var array
+     */
+    private $genericMimeTypes = [
+        'application/x-empty',
+        'inode/x-empty',
+    ];
+
     /**
      * Get mime type of a file
      *
@@ -120,7 +132,11 @@ private function getNativeMimeType(string $file): string
         $extension = $this->getFileExtension($file);
         $result = mime_content_type($file);
         if (isset($this->mimeTypes[$extension], $this->defineByExtensionList[$extension])
-            && (strpos($result, 'text/') === 0 || strpos($result, 'image/svg') === 0)
+            && (
+                strpos($result, 'text/') === 0
+                || strpos($result, 'image/svg') === 0
+                || in_array($result, $this->genericMimeTypes, true)
+            )
         ) {
             $result = $this->mimeTypes[$extension];
         }

lib/internal/Magento/Framework/File/Test/Unit/MimeTest.php

@@ -5,13 +5,19 @@
  */
 namespace Magento\Framework\File\Test\Unit;
 
+/**
+ * Test mime type utility for correct
+ */
 class MimeTest extends \PHPUnit\Framework\TestCase
 {
     /**
      * @var \Magento\Framework\File\Mime
      */
     private $object;
 
+    /**
+     * @inheritDoc
+     */
     protected function setUp()
     {
         $this->object = new \Magento\Framework\File\Mime();
@@ -42,12 +48,13 @@ public function testGetMimeType($file, $expectedType)
     /**
      * @return array
      */
-    public function getMimeTypeDataProvider()
+    public function getMimeTypeDataProvider(): array
     {
         return [
             'javascript' => [__DIR__ . '/_files/javascript.js', 'application/javascript'],
             'weird extension' => [__DIR__ . '/_files/file.weird', 'application/octet-stream'],
             'weird uppercase extension' => [__DIR__ . '/_files/UPPERCASE.WEIRD', 'application/octet-stream'],
+            'generic mime type' => [__DIR__ . '/_files/blank.html', 'text/html'],
         ];
     }
 }