Add documentation for http payload types

[Cali0707]

Parent issue: #223
Depends on: #267, #268, #269, #270, #271

Description

Document all new HTTP payload type options.

Requirements

• Update mcp_file_format.md with all content type options
• Add examples for each payload type:
  • Combined query params + body
  • form-urlencoded
  • multipart/form-data
  • XML
  • text/plain
• Document when to use each type
• Document any limitations or caveats

Acceptance Criteria

• All content types documented in spec
• Examples provided for each
• Usage guidance included