fix: Changed SMTP placeholder for SMTP plugin (#21458)

[sanveer-osahan]

Description

Fixed the attachment placeholder for a query created on SMTP datasource

Fixes #21458

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Manual

For sending email with attachments, tested {{FilePicker.files}} and {{[FilePicker.files[0]]}}, both works successfully but {{FilePicker.files[0]}}

Checklist:

Dev activity

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• PR is being merged under a feature flag

QA activity:

• Test plan has been approved by relevant developers
• Test plan has been peer reviewed by QA
• Cypress test cases have been added and approved by either SDET or manual QA
• Organized project review call with relevant stakeholders after Round 1/2 of QA
• Added Test Plan Approved label after reveiwing all Cypress test

[sanveer-osahan]

/ok-to-test sha=dc36c6cee7b7fab86ee5e330143d0a6f574803eb

[github-actions]

Tests running at: https://github.com/appsmithorg/appsmith/actions/runs/4571645308.
Workflow: Appsmith External Integration Test Workflow.
Commit: dc36c6cee7b7fab86ee5e330143d0a6f574803eb.
PR: 21941.
Perf tests will be available at https://app.appsmith.com/app/performance-infra-dashboard/pr-details-638dd7cd2913ba43778b915e?pr=21941&runId=4571645308_1

[github-actions]

The following are new failures, please fix them before merging the PR:

1. cypress/integration/Regression_TestSuite/ClientSideTests/ExplorerTests/Entity_Explorer_Query_Datasource_spec.js

2. cypress/integration/Regression_TestSuite/ServerSideTests/ApiTests/API_ContextMenu_spec.js