Make snippet filename optional.

Fixes #2384.

app/models/snippet.rb

@@ -34,7 +34,6 @@ 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 },
     format: { with: Gitlab::Regex.file_name_regex,
               message: Gitlab::Regex.file_name_regex_message }

app/views/shared/snippets/_form.html.haml

@@ -18,7 +18,7 @@
         .col-sm-10
           .file-holder.snippet
             .file-title
-              = f.text_field :file_name, placeholder: "example.rb", class: 'form-control snippet-file-name', required: true
+              = f.text_field :file_name, placeholder: "Optionally name this file to add code highlighting, e.g. example.rb for Ruby.", class: 'form-control snippet-file-name'
             .file-content.code
               %pre#editor= @snippet.content
               = f.hidden_field :content, class: 'snippet-file-content'

spec/models/snippet_spec.rb

@@ -40,7 +40,6 @@ describe Snippet do
     it { is_expected.to validate_presence_of(:title) }
     it { is_expected.to validate_length_of(:title).is_within(0..255) }
 
-    it { is_expected.to validate_presence_of(:file_name) }
     it { is_expected.to validate_length_of(:file_name).is_within(0..255) }
 
     it { is_expected.to validate_presence_of(:content) }