fix(nocodb): attachment-related issues

[wingkwong]

Change Summary

• change attachment sample payload to array of objects
• add missing convertAttachmentType in different calls
• return object if incoming data is object, else array of objects.
• ref: 🐛 Bug: Related database is not shown correctly since upgrade from 0.99.0 to 0.99.1 #4500

Change type

• feat: (new feature for the user, not a new feature for build script)
• fix: (bug fix for the user, not a fix to a build script)
• docs: (changes to the documentation)
• style: (formatting, missing semi colons, etc; no production code change)
• refactor: (refactoring production code, eg. renaming a variable)
• test: (adding missing tests, refactoring tests; no production code change)
• chore: (updating grunt tasks etc; no production code change)

Test/ Verification

Provide summary of changes.

Additional information / screenshots (optional)

Anything for maintainers to be made aware of