fix: add PDF upload MIME fallback

[cyphercodes]

Summary

• Add .pdf to the upload MIME fallback map as application/pdf
• Add a focused unit regression test for PDF fallback detection

Testing

• pnpm exec vitest run --project unit packages/payload/src/uploads/getFileTypeFallback.spec.ts
• pnpm exec vitest run --project unit packages/payload/src/uploads
• pnpm exec prettier --check packages/payload/src/uploads/getFileTypeFallback.ts packages/payload/src/uploads/getFileTypeFallback.spec.ts
• pnpm exec eslint --flag v10_config_lookup_from_file --cache --cache-strategy content packages/payload/src/uploads/getFileTypeFallback.ts
• pnpm exec eslint --flag v10_config_lookup_from_file --cache --cache-strategy content --no-warn-ignored packages/payload/src/uploads/getFileTypeFallback.spec.ts
• pnpm --filter payload build:swc
• git diff --check

Fixes #16485