diff --git a/Gemfile b/Gemfile
index 2a37757af..af86a8ba9 100644
--- a/Gemfile
+++ b/Gemfile
@@ -36,6 +36,7 @@ gem 'jquery-rails'
 gem 'webpacker', '>= 4.0.x'
 gem 'retriable'
 gem 'sidekiq'
+gem 'marcel'
 # bookmarks for later, if they are needed
 # gem 'sidekiq-worker-killer'
 gem 'sidekiq-unique-jobs'
@@ -45,7 +46,6 @@ gem 'request_store'
 gem 'elasticsearch-model'
 gem 'elasticsearch-rails'
 
-
 gem 'mailgun-ruby'
 
 # needed for looser jpeg header compat
diff --git a/Gemfile.lock b/Gemfile.lock
index 24e457839..01d7f2366 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -459,6 +459,7 @@ DEPENDENCIES
   jquery-rails
   listen
   mailgun-ruby
+  marcel
   mechanize
   memoist
   minitest-ci
diff --git a/app/logical/upload_service/utils.rb b/app/logical/upload_service/utils.rb
index 9031b1988..01df5085a 100644
--- a/app/logical/upload_service/utils.rb
+++ b/app/logical/upload_service/utils.rb
@@ -6,18 +6,20 @@ class UploadService
     IMAGE_TYPES = %i[original large preview crop]
 
     def file_header_to_file_ext(file)
-      # TODO: marcel?
-      case File.read(file.path, 16)
-      when /^\xff\xd8/n
-        "jpg"
-      when /^GIF87a/, /^GIF89a/
-        "gif"
-      when /^\x89PNG\r\n\x1a\n/n
-        "png"
-      when /^\x1a\x45\xdf\xa3/n
-        "webm"
-      else
-        "bin"
+      File.open file.path do |bin|
+        mime_type = Marcel::MimeType.for(bin)
+        case mime_type
+        when "image/jpeg"
+          "jpg"
+        when "image/gif"
+          "gif"
+        when "image/png"
+          "png"
+        when "video/webm"
+          "webm"
+        else
+          mime_type
+        end
       end
     end
 
@@ -44,11 +46,8 @@ class UploadService
         image_size = ImageSpec.new(file.path)
         yield(image_size.width, image_size.height)
 
-      elsif upload.file_ext == "bin"
-        yield(0, 0)
-
       else
-        raise ArgumentError, "unhandled file type (#{upload.file_ext})" # should not happen
+        yield(0, 0)
       end
     end
 
diff --git a/app/models/post_replacement.rb b/app/models/post_replacement.rb
index 9d18d8603..2fb9d1d2d 100644
--- a/app/models/post_replacement.rb
+++ b/app/models/post_replacement.rb
@@ -38,10 +38,6 @@ class PostReplacement < ApplicationRecord
       %w(jpg jpeg gif png).include?(file_ext)
     end
 
-    def is_flash?
-      %w(swf).include?(file_ext)
-    end
-
     def is_video?
       %w(webm).include?(file_ext)
     end
@@ -114,8 +110,8 @@ class PostReplacement < ApplicationRecord
 
     def update_file_attributes
       self.file_ext = UploadService::Utils.file_header_to_file_ext(replacement_file)
-      if file_ext == "bin"
-        self.errors.add(:base, "Unknown or invalid file format")
+      if Danbooru.config.max_file_sizes.keys.exclude? file_ext
+        self.errors.add(:base, "Unknown or invalid file format: #{file_ext}")
         throw :abort
       end
       self.file_size = replacement_file.size
diff --git a/app/models/upload.rb b/app/models/upload.rb
index 0b7a09516..5d8a2bf25 100644
--- a/app/models/upload.rb
+++ b/app/models/upload.rb
@@ -15,14 +15,15 @@ class Upload < ApplicationRecord
     end
 
     def validate_file_integrity(record)
-      if record.file_ext.in?(["jpg", "jpeg", "gif", "png"]) && DanbooruImageResizer.is_corrupt?(record.file.path)
+      if record.is_image? && DanbooruImageResizer.is_corrupt?(record.file.path)
         record.errors[:file] << "File is corrupt"
       end
     end
 
     def validate_file_ext(record)
-      if record.file_ext == "bin"
-        record.errors.add(:file_ext, "is invalid (only JPEG, PNG, GIF, and WebM files are allowed")
+      if Danbooru.config.max_file_sizes.keys.exclude? record.file_ext
+        record.errors.add(:file_ext, "#{record.file_ext} is invalid (only JPEG, PNG, GIF, and WebM files are allowed")
+        throw :abort
       end
     end
 
@@ -118,10 +119,6 @@ class Upload < ApplicationRecord
       %w(jpg jpeg gif png).include?(file_ext)
     end
 
-    def is_flash?
-      %w(swf).include?(file_ext)
-    end
-
     def is_video?
       %w(webm).include?(file_ext)
     end
diff --git a/config/danbooru_default_config.rb b/config/danbooru_default_config.rb
index aff1f4237..27131cd13 100644
--- a/config/danbooru_default_config.rb
+++ b/config/danbooru_default_config.rb
@@ -375,13 +375,10 @@ fart'
 
     def max_file_sizes
       {
-          'jpg' => 100.megabytes,
-          'gif' => 20.megabytes,
-          'png' => 100.megabytes,
-          'swf' => 0,
-          'webm' => 100.megabytes,
-          'mp4' => 100.megabytes,
-          'zip' => 0
+        'jpg' => 100.megabytes,
+        'gif' => 20.megabytes,
+        'png' => 100.megabytes,
+        'webm' => 100.megabytes
       }
     end