diff --git a/app/models/attachment.rb b/app/models/attachment.rb
index 12d4c809c..eed654f14 100644
--- a/app/models/attachment.rb
+++ b/app/models/attachment.rb
@@ -227,7 +227,7 @@ class Attachment < ApplicationRecord
   end
 
   def image?
-    !!(self.filename =~ /\.(bmp|gif|jpg|jpe|jpeg|png|webp)$/i)
+    !!(self.filename =~ /\.(avif|bmp|gif|jpg|jpe|jpeg|png|webp)$/i)
   end
 
   def thumbnailable?
diff --git a/lib/redmine/mime_type.rb b/lib/redmine/mime_type.rb
index 5554cc5a9..170264715 100644
--- a/lib/redmine/mime_type.rb
+++ b/lib/redmine/mime_type.rb
@@ -39,6 +39,7 @@ module Redmine
       'text/yaml' => 'yml,yaml',
       'text/csv' => 'csv',
       'text/x-po' => 'po',
+      'image/avif' => 'avif',
       'image/gif' => 'gif',
       'image/jpeg' => 'jpg,jpeg,jpe',
       'image/png' => 'png',
diff --git a/lib/redmine/thumbnail.rb b/lib/redmine/thumbnail.rb
index 556680a5a..df1612dc8 100644
--- a/lib/redmine/thumbnail.rb
+++ b/lib/redmine/thumbnail.rb
@@ -30,7 +30,7 @@ module Redmine
       ('gswin64c' if Redmine::Platform.mswin?) ||
       'gs'
     ).freeze
-    ALLOWED_TYPES = %w(image/bmp image/gif image/jpeg image/png image/webp application/pdf)
+    ALLOWED_TYPES = %w(image/avif image/bmp image/gif image/jpeg image/png image/webp application/pdf)
 
     # Generates a thumbnail for the source image to target
     # TODO: Remove the deprecated _is_pdf parameter in Redmine 7.0
diff --git a/lib/redmine/wiki_formatting/inline_attachments_scrubber.rb b/lib/redmine/wiki_formatting/inline_attachments_scrubber.rb
index 2da380df7..64b0cc88e 100644
--- a/lib/redmine/wiki_formatting/inline_attachments_scrubber.rb
+++ b/lib/redmine/wiki_formatting/inline_attachments_scrubber.rb
@@ -51,7 +51,7 @@ module Redmine
 
         src = node['src']
 
-        if src =~ %r{\A(?<filename>[^/"]+?\.(?:bmp|gif|jpg|jpeg|jpe|png|webp))\z}i
+        if src =~ %r{\A(?<filename>[^/"]+?\.(?:avif|bmp|gif|jpg|jpeg|jpe|png|webp))\z}i
           filename = $~[:filename]
           if found = find_attachment(CGI.unescape(filename))
             image_url = @view.download_named_attachment_url(found, found.filename, :only_path => @only_path)
diff --git a/test/unit/attachment_test.rb b/test/unit/attachment_test.rb
index 1af8eb0f2..387da73c5 100644
--- a/test/unit/attachment_test.rb
+++ b/test/unit/attachment_test.rb
@@ -520,6 +520,7 @@ class AttachmentTest < ActiveSupport::TestCase
 
   def test_thumbnailable_should_be_true_for_images
     skip unless convert_installed?
+    assert_equal true, Attachment.new(:filename => 'test.avif').thumbnailable?
     assert_equal true, Attachment.new(:filename => 'test.jpg').thumbnailable?
     assert_equal true, Attachment.new(:filename => 'test.webp').thumbnailable?
   end
diff --git a/test/unit/lib/redmine/mime_type_test.rb b/test/unit/lib/redmine/mime_type_test.rb
index 95aaef034..265146462 100644
--- a/test/unit/lib/redmine/mime_type_test.rb
+++ b/test/unit/lib/redmine/mime_type_test.rb
@@ -24,6 +24,7 @@ class Redmine::MimeTypeTest < ActiveSupport::TestCase
     to_test = {
       'test.txt' => 'text/plain',
       'test.c' => 'text/x-c',
+      'test.avif' => 'image/avif',
       'TEST.JPG' => 'image/jpeg',
     }
     to_test.each do |name, expected|
@@ -79,6 +80,7 @@ class Redmine::MimeTypeTest < ActiveSupport::TestCase
 
   def test_by_type
     image_types = Redmine::MimeType.by_type('image')
+    assert_includes image_types, 'image/avif'
     assert_includes image_types, 'image/png'
     assert_includes image_types, 'image/webp'
   end
diff --git a/test/unit/lib/redmine/wiki_formatting/inline_attachments_scrubber_test.rb b/test/unit/lib/redmine/wiki_formatting/inline_attachments_scrubber_test.rb
index 2c441c0e5..278a4c7c5 100644
--- a/test/unit/lib/redmine/wiki_formatting/inline_attachments_scrubber_test.rb
+++ b/test/unit/lib/redmine/wiki_formatting/inline_attachments_scrubber_test.rb
@@ -35,6 +35,7 @@ class Redmine::WikiFormatting::InlineAttachmentsScrubberTest < Redmine::HelperTe
 
   def test_should_resolve_attached_images
     to_test = [
+      "test.avif",
       "test.bmp",
       "test.gif",
       "test.jpg",