diff --git a/Gemfile b/Gemfile
index 36595d2b2cf..9b906b676d4 100644
--- a/Gemfile
+++ b/Gemfile
@@ -115,8 +115,7 @@ gem 'connection_pool'
 gem 'dalli'
 gem 'kgio', '2.10.0'
 
-# TODO: AO3-6297 Update the download code so we can remove mimemagic.
-gem "mimemagic", "0.3.10"
+gem "marcel", "1.0.2"
 
 # Library for helping run pt-online-schema-change commands:
 gem "departure", "~> 6.5"
diff --git a/Gemfile.lock b/Gemfile.lock
index ca00b349823..7fcdc79b48e 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -351,11 +351,8 @@ GEM
     mime-types (3.4.1)
       mime-types-data (~> 3.2015)
     mime-types-data (3.2022.0105)
-    mimemagic (0.3.10)
-      nokogiri (~> 1)
-      rake
-    mini_mime (1.1.5)
-    mini_portile2 (2.8.5)
+    mini_mime (1.1.2)
+    mini_portile2 (2.8.2)
     minitest (5.17.0)
     mono_logger (1.1.2)
     multi_json (1.15.0)
@@ -380,22 +377,21 @@ GEM
       net-ssh (>= 2.6.5, < 7.0.0)
     net-sftp (3.0.0)
       net-ssh (>= 5.0.0, < 7.0.0)
-    net-smtp (0.4.0)
+    net-smtp (0.5.0)
       net-protocol
     net-ssh (6.1.0)
     net-ssh-gateway (2.0.0)
       net-ssh (>= 4.0.0)
     netrc (0.11.0)
-    newrelic_rpm (9.7.1)
+    newrelic_rpm (8.16.0)
     nio4r (2.5.8)
-    nokogiri (1.16.2)
-      mini_portile2 (~> 2.8.2)
+    nokogiri (1.14.3)
+      mini_portile2 (~> 2.8.0)
       racc (~> 1.4)
     orm_adapter (0.5.0)
     parallel (1.23.0)
-    parser (3.3.0.5)
+    parser (3.1.0.0)
       ast (~> 2.4.1)
-      racc
     permit_yo (2.1.3)
     phraseapp-in-context-editor-ruby (1.4.0)
       i18n (>= 0.6)
@@ -417,7 +413,7 @@ GEM
     pundit (2.1.1)
       activesupport (>= 3.0.0)
     raabro (1.4.0)
-    racc (1.7.3)
+    racc (1.7.1)
     rack (2.2.6.4)
     rack-attack (6.6.0)
       rack (>= 1.0, < 3)
@@ -673,8 +669,8 @@ DEPENDENCIES
   listen (~> 3.3)
   lograge
   mail (>= 2.8)
+  marcel (= 1.0.2)
   mechanize
-  mimemagic (= 0.3.10)
   minitest
   mysql2
   n_plus_one_control
@@ -727,4 +723,4 @@ RUBY VERSION
    ruby 3.1.4p223
 
 BUNDLED WITH
-   2.3.26
+   2.2.33
diff --git a/app/models/download.rb b/app/models/download.rb
index d7d2c799f4c..e05af505292 100644
--- a/app/models/download.rb
+++ b/app/models/download.rb
@@ -4,9 +4,7 @@ class Download
   def initialize(work, options = {})
     @work = work
     @file_type = set_file_type(options.slice(:mime_type, :format))
-    # TODO: Our current version of the mime-types gem doesn't include azw3, but
-    # the gem cannot be updated without updating rest-client
-    @mime_type = @file_type == "azw3" ? "application/x-mobi8-ebook" : MIME::Types.type_for(@file_type).first
+    @mime_type = Marcel::MimeType.for(extension: @file_type).to_s
     @include_draft_chapters = options[:include_draft_chapters]
   end
 
@@ -40,16 +38,17 @@ def set_file_type(options)
 
   # Given a mime type, return a file extension
   def file_type_from_mime(mime)
-    ext = MimeMagic.new(mime.to_s).subtype
-    case ext
+    subtype = Marcel::Magic.new(mime.to_s).subtype
+    case subtype
     when "x-mobipocket-ebook"
       "mobi"
     when "x-mobi8-ebook"
       "azw3"
     else
-      ext
+      subtype
     end
   end
+  
 
   # The base name of the file (e.g., "War_and_Peace")
   def file_name
diff --git a/config/initializers/mime_types.rb b/config/initializers/mime_types.rb
index bcd3e9d6106..4fac7566ca9 100644
--- a/config/initializers/mime_types.rb
+++ b/config/initializers/mime_types.rb
@@ -1,13 +1,9 @@
-# Be sure to restart your server when you modify this file.
-
-# Add new mime types for use in respond_to blocks:
-# Mime::Type.register "text/richtext", :rtf
-
 # for azw3 files
-Mime::Type.register 'application/x-mobi8-ebook', :azw3
-
-# for mobi files
-Mime::Type.register 'application/x-mobipocket-ebook', :mobi
-
+Marcel::MimeType.extend "application/x-mobi8-ebook", extensions: %w[azw3]
+Mime::Type.register "application/x-mobi8-ebook", :azw3
 # for epub files
-Mime::Type.register 'application/epub', :epub
+Marcel::MimeType.extend "application/epub", extensions: %w[epub]
+Mime::Type.register "application/epub", :epub
+
+# for mobi type (already present in marcel types)
+Mime::Type.register "application/x-mobipocket-ebook", :mobi
diff --git a/features/step_definitions/work_download_steps.rb b/features/step_definitions/work_download_steps.rb
index 32e61e7cdb6..bf32c3e6981 100644
--- a/features/step_definitions/work_download_steps.rb
+++ b/features/step_definitions/work_download_steps.rb
@@ -11,7 +11,7 @@
 end
 
 Then /^I should receive a file of type "(.*?)"$/ do |filetype|
-  mime_type = filetype == "azw3" ? "application/x-mobi8-ebook" : MIME::Types.type_for("foo.#{filetype}").first
+  mime_type = Marcel::MimeType.for(name: "foo.#{filetype}").to_s
   expect(page.response_headers['Content-Disposition']).to match(/filename=.+\.#{filetype}/)
   expect(page.response_headers['Content-Length'].to_i).to be_positive
   expect(page.response_headers['Content-Type']).to eq(mime_type)
@@ -30,7 +30,7 @@
 
   download = Download.new(work, format: filetype)
   filename = "#{download.file_name}.#{download.file_type}"
-  mime_type = filetype == "azw3" ? "application/x-mobi8-ebook" : MIME::Types.type_for(filename).first
+  mime_type = Marcel::MimeType.for(name: filename).to_s
   expect(page.response_headers['Content-Disposition']).to match(/filename="#{filename}"/)
   expect(page.response_headers['Content-Length'].to_i).to be_positive
   expect(page.response_headers['Content-Type']).to eq(mime_type)