[Feature]: Add filesystem config contentTypeCharset support

[baichangda]

Description

add filesystem config contentTypeCharset support

Fixes #2434

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• 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 - /docs/api/middleware/filesystem.md directory for https://docs.gofiber.io/
• 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

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