Add strict validation to snippet file names

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/snippet.rb

@@ -29,7 +29,9 @@ class Snippet < ActiveRecord::Base
 
   validates :author, presence: true
   validates :title, presence: true, length: { within: 0..255 }
-  validates :file_name, presence: true, length: { within: 0..255 }
+  validates :file_name, presence: true, length: { within: 0..255 },
+            format: { with: Gitlab::Regex.path_regex,
+                      message: Gitlab::Regex.path_regex_message }
   validates :content, presence: true
   validates :visibility_level, inclusion: { in: Gitlab::VisibilityLevel.values }