fix: prevent selecting duplicate files in the same modal session

[andrewleith]

Summary | Résumé

This pull request improves the file attachment modal by preventing users from selecting the same file more than once in a single session. It also adds a test to verify that duplicate files are detected and reported as issues.

Test instructions | Instructions pour tester la modification

Prevents duplicates between reloads

• Go to an email template, click attach more files
• Click choose files, select a file and attach it
• Reload the page
• Attach the same file
• An error message is triggered and the file is not attached

Prevents duplicates done one at a time

• Go to an email template, click attach more files
• Click choose files, select a file and attach it
• Click attach more files again
• Select the same file and attach it again
• An error message is triggered and the file is not attached

Prevents duplicates done at the same time

• Go to an email template, click attach more files
• Click choose files, select a file and attach it
• Click choose files, select the same file and attach it again
• An error message is triggered and the file is not attached

[Copilot]

Pull request overview

This PR enhances the attachments UI by blocking users from selecting the same file more than once during a single “Attach files” modal session, and adds a regression test to ensure duplicates are detected and reported.

Changes:

• Add in-modal duplicate detection (against already-selected pending files) and filter duplicates out of the accepted selection.
• Deduplicate reported validation issues before surfacing them to the parent via onIssuesChange.
• Add a Jest test covering duplicate selection within the same modal session.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
app/assets/javascripts/attachments/AttachFilesModal.js	Filters newly selected files to prevent duplicates against the modal’s existing pending selection and reports a duplicate issue.
tests/javascripts/attachments/AttachFilesModal.test.js	Adds a test verifying duplicate selection triggers an issue and does not add a second pending item.

Comments suppressed due to low confidence (1)

app/assets/javascripts/attachments/AttachFilesModal.js:177

• After handling a file selection, the file input should be cleared (set to an empty string) so choosing the same file again reliably fires a change event across browsers. Without this, the duplicate-selection validation may not run if the user re-selects the same file.

    const normalizedFiles = filteredAcceptedFiles.map((file) => ({
      id: `${file.name}-${file.size}-${file.lastModified || 0}-${Math.random()}`,
      file,
    }));
    setSelectedFiles((currentFiles) => [...currentFiles, ...normalizedFiles]);
  };

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