--- a/app/models/attachment.rb
+++ b/app/models/attachment.rb
@@ -220,7 +220,7 @@
   end

   def image?
-    !!(self.filename =~ /\.(bmp|gif|jpg|jpe|jpeg|png|webp)$/i)
+    !!(self.filename =~ /\.(bmp|gif|jpg|jpe|jpeg|png|webp|heic|heif)$/i)
   end

   def thumbnailable?
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -999,7 +999,7 @@
     if attachments.present?
       title_and_alt_re = /\s+(title|alt)="([^"]*)"/i

-      text.gsub!(/src="([^\/"]+\.(bmp|gif|jpg|jpe|jpeg|png|webp))"([^>]*)/i) do |m|
+      text.gsub!(/src="([^\/"]+\.(bmp|gif|jpg|jpe|jpeg|png|webp|heic|heif))"([^>]*)/i) do |m|
         filename, ext, other_attrs = $1, $2, $3

         # search for the picture in attachments
--- a/lib/redmine/thumbnail.rb
+++ b/lib/redmine/thumbnail.rb
@@ -30,7 +30,7 @@
       ('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/bmp image/gif image/jpeg image/png image/webp image/heic image/heif application/pdf)

     # Generates a thumbnail for the source image to target
     # TODO: Remove the deprecated _is_pdf parameter in Redmine 7.0
@@ -51,6 +51,8 @@
           return nil unless valid_pdf_magic?(source)

           cmd = "#{shell_quote CONVERT_BIN} #{shell_quote "#{source}[0]"} -thumbnail #{shell_quote size_option} #{shell_quote "png:#{target}"}"
+        elsif mime_type == 'image/heic' || mime_type == 'image/heif'
+          cmd = "#{shell_quote CONVERT_BIN} #{shell_quote source} -auto-orient -thumbnail #{shell_quote size_option} #{shell_quote "jpg:#{target}"}"
         else
           cmd = "#{shell_quote CONVERT_BIN} #{shell_quote source} -auto-orient -thumbnail #{shell_quote size_option} #{shell_quote target}"
         end
--- a/app/controllers/attachments_controller.rb
+++ b/app/controllers/attachments_controller.rb
@@ -294,6 +294,9 @@
     if is_thumb && content_type == "application/pdf"
       # PDF previews are stored in PNG format
       content_type = "image/png"
+    elsif is_thumb && (content_type == "image/heic" || content_type == "image/heif")
+      # HEIC/HEIF previews are stored in JPEG format
+      content_type = "image/jpeg"
     end

     content_type