Fix file name extraction.

src/Validation/Validation.php

@@ -1187,7 +1187,11 @@ class Validation
             return $check['tmp_name'];
         }
 
-        return $check;
+        if (is_string($check)) {
+            return $check;
+        }
+
+        return false;
     }
 
     /**

tests/TestCase/Validation/ValidationTest.php

@@ -2520,6 +2520,7 @@ class ValidationTest extends TestCase
 
         $this->assertFalse(Validation::mimeType($image, ['image/png']));
         $this->assertFalse(Validation::mimeType(['tmp_name' => $image], ['image/png']));
+        $this->assertFalse(Validation::mimeType([], ['image/png']));
     }
 
     /**