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 }
@@ -72,7 +74,7 @@ class Snippet < ActiveRecord::Base
   def visibility_level_field
     visibility_level
-  end 
+  end
   class << self
     def search(query)