feat(File): Add missing return type declarations

[usernane]

Problem Statement

Several methods lack return type declarations despite the library requiring PHP 8.0+:

• getID() — returns string|int (no type declared)
• getLastModified() — returns string|int (no type declared)
• read() — returns void (no type declared)
• readDecoded() — returns void (no type declared)
• setId() — returns void (no type declared)
• setMIME() — returns void (no type declared)
• setName() — returns void (no type declared)

Proposed Solution

Add proper return type declarations using PHP 8.0 union types where needed (e.g., string|int).

Alternatives Considered

• Using mixed return type (less precise)

Breaking Change

No — adding return types is backward compatible.