diff --git a/app/controllers/api/v1/statuses_controller.rb b/app/controllers/api/v1/statuses_controller.rb
index 69cbdce5d..552f1b1b3 100644
--- a/app/controllers/api/v1/statuses_controller.rb
+++ b/app/controllers/api/v1/statuses_controller.rb
@@ -67,7 +67,6 @@ class Api::V1::StatusesController < ApiController
                                                                                                                                                              spoiler_text: params[:spoiler_text],
                                                                                                                                                              visibility: params[:visibility],
                                                                                                                                                              application: doorkeeper_token.application)
-
     render action: :show
   end
 
diff --git a/app/controllers/api_controller.rb b/app/controllers/api_controller.rb
index 5d2bd9a22..c2002cb79 100644
--- a/app/controllers/api_controller.rb
+++ b/app/controllers/api_controller.rb
@@ -10,7 +10,7 @@ class ApiController < ApplicationController
 
   before_action :set_rate_limit_headers
 
-  rescue_from ActiveRecord::RecordInvalid do |e|
+  rescue_from ActiveRecord::RecordInvalid, Mastodon::ValidationError do |e|
     render json: { error: e.to_s }, status: 422
   end
 
@@ -30,7 +30,7 @@ class ApiController < ApplicationController
     render json: { error: 'Remote SSL certificate could not be verified' }, status: 503
   end
 
-  rescue_from Mastodon::NotPermitted do
+  rescue_from Mastodon::NotPermittedError do
     render json: { error: 'This action is not allowed' }, status: 403
   end
 
diff --git a/app/controllers/authorize_follow_controller.rb b/app/controllers/authorize_follow_controller.rb
index e866b5599..c98a5f45f 100644
--- a/app/controllers/authorize_follow_controller.rb
+++ b/app/controllers/authorize_follow_controller.rb
@@ -25,7 +25,7 @@ class AuthorizeFollowController < ApplicationController
     else
       redirect_to web_url("accounts/#{@account.id}")
     end
-  rescue ActiveRecord::RecordNotFound, Mastodon::NotPermitted
+  rescue ActiveRecord::RecordNotFound, Mastodon::NotPermittedError
     render :error
   end
 
diff --git a/app/lib/exceptions.rb b/app/lib/exceptions.rb
index 359228c29..200da9fe1 100644
--- a/app/lib/exceptions.rb
+++ b/app/lib/exceptions.rb
@@ -2,5 +2,6 @@
 
 module Mastodon
   class Error < StandardError; end
-  class NotPermitted < Error; end
+  class NotPermittedError < Error; end
+  class ValidationError < Error; end
 end
diff --git a/app/services/favourite_service.rb b/app/services/favourite_service.rb
index 818898302..5cc96403c 100644
--- a/app/services/favourite_service.rb
+++ b/app/services/favourite_service.rb
@@ -6,7 +6,7 @@ class FavouriteService < BaseService
   # @param [Status] status
   # @return [Favourite]
   def call(account, status)
-    raise Mastodon::NotPermitted unless status.permitted?(account)
+    raise Mastodon::NotPermittedError unless status.permitted?(account)
 
     favourite = Favourite.create!(account: account, status: status)
 
diff --git a/app/services/follow_service.rb b/app/services/follow_service.rb
index 915f95b4c..17b3b2542 100644
--- a/app/services/follow_service.rb
+++ b/app/services/follow_service.rb
@@ -10,7 +10,7 @@ class FollowService < BaseService
     target_account = FollowRemoteAccountService.new.call(uri)
 
     raise ActiveRecord::RecordNotFound if target_account.nil? || target_account.id == source_account.id || target_account.suspended?
-    raise Mastodon::NotPermitted       if target_account.blocking?(source_account) || source_account.blocking?(target_account)
+    raise Mastodon::NotPermittedError       if target_account.blocking?(source_account) || source_account.blocking?(target_account)
 
     if target_account.locked?
       request_follow(source_account, target_account)
diff --git a/app/services/post_status_service.rb b/app/services/post_status_service.rb
index 979941c84..b8179f7dc 100644
--- a/app/services/post_status_service.rb
+++ b/app/services/post_status_service.rb
@@ -13,6 +13,7 @@ class PostStatusService < BaseService
   # @option [Doorkeeper::Application] :application
   # @return [Status]
   def call(account, text, in_reply_to = nil, options = {})
+    media  = validate_media!(options[:media_ids])
     status = account.statuses.create!(text: text,
                                       thread: in_reply_to,
                                       sensitive: options[:sensitive],
@@ -20,7 +21,7 @@ class PostStatusService < BaseService
                                       visibility: options[:visibility],
                                       application: options[:application])
 
-    attach_media(status, options[:media_ids])
+    attach_media(status, media)
     process_mentions_service.call(status)
     process_hashtags_service.call(status)
 
@@ -33,10 +34,20 @@ class PostStatusService < BaseService
 
   private
 
-  def attach_media(status, media_ids)
+  def validate_media!(media_ids)
     return if media_ids.nil? || !media_ids.is_a?(Enumerable)
 
+    raise Mastodon::ValidationError, 'Cannot attach more than 4 files' if media_ids.size > 4
+
     media = MediaAttachment.where(status_id: nil).where(id: media_ids.take(4).map(&:to_i))
+
+    raise Mastodon::ValidationError, 'Cannot attach a video to a toot that already contains images' if media.size > 1 && media.find(&:video?)
+
+    media
+  end
+
+  def attach_media(status, media)
+    return if media.nil?
     media.update(status_id: status.id)
   end
 
diff --git a/app/services/reblog_service.rb b/app/services/reblog_service.rb
index 7a52f041f..c14b2925a 100644
--- a/app/services/reblog_service.rb
+++ b/app/services/reblog_service.rb
@@ -10,7 +10,7 @@ class ReblogService < BaseService
   def call(account, reblogged_status)
     reblogged_status = reblogged_status.reblog if reblogged_status.reblog?
 
-    raise Mastodon::NotPermitted if reblogged_status.private_visibility? || !reblogged_status.permitted?(account)
+    raise Mastodon::NotPermittedError if reblogged_status.private_visibility? || !reblogged_status.permitted?(account)
 
     reblog = account.statuses.create!(reblog: reblogged_status, text: '')