diff --git a/app/models/attachment.rb b/app/models/attachment.rb
index c9283ae..19f30a6 100644
--- a/app/models/attachment.rb
+++ b/app/models/attachment.rb
@@ -354,22 +354,37 @@ class Attachment < ActiveRecord::Base
     end
   end
 
-  # Returns true if the extension is allowed, otherwise false
+  # Returns true if the extension is allowed regarding allowed/denied
+  # extensions defined in application settings, otherwise false
   def self.valid_extension?(extension)
-    extension = extension.downcase.sub(/\A\.+/, '')
-
     denied, allowed = [:attachment_extensions_denied, :attachment_extensions_allowed].map do |setting|
-      Setting.send(setting).to_s.split(",").map {|s| s.strip.downcase.sub(/\A\.+/, '')}.reject(&:blank?)
+      Setting.send(setting)
     end
-    if denied.present? && denied.include?(extension)
+    if denied.present? && extension_in?(extension, denied)
       return false
     end
-    unless allowed.blank? || allowed.include?(extension)
+    if allowed.present? && !extension_in?(extension, allowed)
       return false
     end
     true
   end
 
+  # Returns true if extension belongs to extensions list.
+  def self.extension_in?(extension, extensions)
+    extension = extension.downcase.sub(/\A\.+/, '')
+
+    unless extensions.is_a?(Array)
+      extensions = extensions.to_s.split(",").map(&:strip)
+    end
+    extensions = extensions.map {|s| s.downcase.sub(/\A\.+/, '')}.reject(&:blank?)
+    extensions.include?(extension)
+  end
+
+  # Returns true if attachment's extension belongs to extensions list.
+  def extension_in?(extensions)
+    self.class.extension_in?(File.extname(filename), extensions)
+  end
+
   private
 
   # Physically deletes the file from the file system
diff --git a/app/models/custom_field.rb b/app/models/custom_field.rb
index 7f37ec1..46d4581 100644
--- a/app/models/custom_field.rb
+++ b/app/models/custom_field.rb
@@ -87,7 +87,8 @@ class CustomField < ActiveRecord::Base
     'text_formatting',
     'edit_tag_style',
     'user_role',
-    'version_status'
+    'version_status',
+    'extensions_allowed'
 
   def format
     @format ||= Redmine::FieldFormat.find(field_format)
diff --git a/app/views/custom_fields/formats/_attachment.html.erb b/app/views/custom_fields/formats/_attachment.html.erb
index e69de29..263238a 100644
--- a/app/views/custom_fields/formats/_attachment.html.erb
+++ b/app/views/custom_fields/formats/_attachment.html.erb
@@ -0,0 +1,4 @@
+<p>
+  <%= f.text_field :extensions_allowed, :size => 50, :label => :setting_attachment_extensions_allowed %>
+  <em class="info"><%= l(:text_comma_separated) %> <%= l(:label_example) %>: txt, png</em>
+</p>
diff --git a/lib/redmine/field_format.rb b/lib/redmine/field_format.rb
index c2a02d0..7d6502b 100644
--- a/lib/redmine/field_format.rb
+++ b/lib/redmine/field_format.rb
@@ -861,6 +861,7 @@ module Redmine
       self.form_partial = 'custom_fields/formats/attachment'
       self.is_filter_supported = false
       self.change_no_details = true
+      field_attributes :extensions_allowed
 
       def set_custom_field_value(custom_field, custom_field_value, value)
         attachment_present = false
@@ -917,8 +918,18 @@ module Redmine
       def validate_custom_value(custom_value)
         errors = []
 
-        if custom_value.instance_variable_get("@attachment_present") && custom_value.value.blank?
-          errors << ::I18n.t('activerecord.errors.messages.invalid')
+        if custom_value.value.blank?
+          if custom_value.instance_variable_get("@attachment_present")
+            errors << ::I18n.t('activerecord.errors.messages.invalid')
+          end
+        else
+          if custom_value.value.present?
+            attachment = Attachment.where(:id => custom_value.value.to_s).first
+            extensions = custom_value.custom_field.extensions_allowed
+            if attachment && extensions.present? && !attachment.extension_in?(extensions)
+              errors << "#{::I18n.t('activerecord.errors.messages.invalid')} (#{l(:setting_attachment_extensions_allowed)}: #{extensions})"
+            end
+          end
         end
 
         errors.uniq
diff --git a/test/integration/lib/redmine/field_format/attachment_format_test.rb b/test/integration/lib/redmine/field_format/attachment_format_test.rb
index 447ba68..98aa005 100644
--- a/test/integration/lib/redmine/field_format/attachment_format_test.rb
+++ b/test/integration/lib/redmine/field_format/attachment_format_test.rb
@@ -153,4 +153,42 @@ class AttachmentFieldFormatTest < Redmine::IntegrationTest
     assert_equal attachment.id.to_s, custom_value.value
     assert_equal custom_value, attachment.reload.container
   end
+
+  def test_create_with_valid_extension
+    @field.extensions_allowed = "txt, log"
+    @field.save!
+
+    attachment = new_record(Attachment) do
+      assert_difference 'Issue.count' do
+        post '/projects/ecookbook/issues', {
+            :issue => {
+              :subject => "Blank",
+              :custom_field_values => {
+                @field.id => {:file => uploaded_test_file("testfile.txt", "text/plain")}
+              }
+            }
+          }
+        assert_response 302
+      end
+    end
+  end
+
+  def test_create_with_invalid_extension_should_fail
+    @field.extensions_allowed = "png, jpeg"
+    @field.save!
+
+    attachment = new_record(Attachment) do
+      assert_no_difference 'Issue.count' do
+        post '/projects/ecookbook/issues', {
+            :issue => {
+              :subject => "Blank",
+              :custom_field_values => {
+                @field.id => {:file => uploaded_test_file("testfile.txt", "text/plain")}
+              }
+            }
+          }
+        assert_response :success
+      end
+    end
+  end
 end