Improve design filename validation error message

Previously this error message was output by Rails as:

  Filename Only these extensions are supported: png, jpg, jpeg, gif,
  bmp, tiff, and ico.

This changes the message to be:

  Filename has an unsupported extension. The extension must be png, jpg,
  jpeg, gif, bmp, tiff, or ico

https://gitlab.com/gitlab-org/gitlab/merge_requests/21615#note_261398477

ee/app/models/design_management/design.rb

@@ -147,7 +147,7 @@ module DesignManagement
 
     def validate_file_is_image
       unless image? || (dangerous_image? && allow_dangerous_images?)
-        message = _("Only these extensions are supported: %{extension_list}") % {
+        message = _('does not have a supported extension. Only %{extension_list} are supported') % {
           extension_list: valid_file_extensions.to_sentence
         }
         errors.add(:filename, message)

ee/changelogs/unreleased/ld-improve-design-filename-validation-error.yml

+---
+title: Improve validation message when a design extension is unsupported
+merge_request: 21888
+author:
+type: changed

ee/spec/models/design_management/design_spec.rb

@@ -35,7 +35,7 @@ describe DesignManagement::Design do
 
       expect(design).not_to be_valid
       expect(design.errors[:filename].first).to eq(
-        "Only these extensions are supported: #{extensions.to_sentence}"
+        "does not have a supported extension. Only #{extensions.to_sentence} are supported"
       )
     end
 
@@ -55,7 +55,7 @@ describe DesignManagement::Design do
 
         expect(design).not_to be_valid
         expect(design.errors[:filename].first).to eq(
-          "Only these extensions are supported: #{described_class::SAFE_IMAGE_EXT.to_sentence}"
+          "does not have a supported extension. Only #{described_class::SAFE_IMAGE_EXT.to_sentence} are supported"
         )
       end
     end

locale/gitlab.pot

@@ -12492,9 +12492,6 @@ msgstr ""
 msgid "Only project members will be imported. Group members will be skipped."
 msgstr ""
 
-msgid "Only these extensions are supported: %{extension_list}"
-msgstr ""
-
 msgid "Only users with an email address in this domain can be added to the group.<br>Example: <code>gitlab.com</code>. Some common domains are not allowed. %{read_more_link}."
 msgstr ""
 
@@ -21707,6 +21704,9 @@ msgstr ""
 msgid "disabled"
 msgstr ""
 
+msgid "does not have a supported extension. Only %{extension_list} are supported"
+msgstr ""
+
 msgid "done"
 msgstr ""