Fix max attachment size for CSV export emails

Changes the email text to use the actual size of the
file and not the base64 encoded file size

doc/user/project/issues/csv_export.md

@@ -72,4 +72,4 @@ Data will be encoded with a comma as the column delimiter, with `"` used to quot
 
 ## Limitations
 
-As the issues will be sent as an email attachment, there is a limit on how much data can be exported. Currently this limit is 20MB to ensure successful delivery across a range of email providers. If this limit is reached we suggest narrowing the search before export, perhaps by exporting open and closed issues separately.
+As the issues will be sent as an email attachment, there is a limit on how much data can be exported. Currently this limit is 15MB to ensure successful delivery across a range of email providers. If this limit is reached we suggest narrowing the search before export, perhaps by exporting open and closed issues separately.

ee/app/views/notify/issues_csv_email.html.haml

@@ -5,4 +5,4 @@
   has been added to this email as an attachment.
   - if @truncated
     %p
-      This attachment has been truncated to avoid exceeding a maximum allowed attachment size of 20MB. #{ @written_count } of #{ @issues_count } issues have been included. Consider re-exporting with a narrower selection of issues.
+      This attachment has been truncated to avoid exceeding a maximum allowed attachment size of 15MB. #{ @written_count } of #{ @issues_count } issues have been included. Consider re-exporting with a narrower selection of issues.

ee/app/views/notify/issues_csv_email.text.erb

 Your CSV export of <%= pluralize(@written_count, 'issue') %> from project <%= @project.full_name %> (<%= project_url(@project) %>) has been added to this email as an attachment.
 
 <% if @truncated %>
-This attachment has been truncated to avoid exceeding a maximum allowed attachment size of 20MB. <%= @written_count %> of <%= @issues_count %> issues have been included. Consider re-exporting with a narrower selection of issues.
-<% end %>
\ No newline at end of file
+This attachment has been truncated to avoid exceeding a maximum allowed attachment size of 15MB. <%= @written_count %> of <%= @issues_count %> issues have been included. Consider re-exporting with a narrower selection of issues.
+<% end %>

ee/changelogs/unreleased/10726-exporting-issues-size-is-15mb-but-max-is-20mb.yml

+---
+title: Fix max attachment size used in CSV export email messages
+merge_request: 14884
+author:
+type: fixed