📚 Doc: Clarify SendFile Docs

[xEricL]

Description

These changes include:

• Adds fiber. to SendFile for the SendFile config parameter in the docs.
• Clarifies that the ContentType header is set based on the format of the file if the file extension is missing/invalid.
• Adds unit-tests to verify the ContentType header is set as described.
• Makes the formatting of default values for SendFile struct fields consistent (matching the FS field and most middleware configs.
• Adds missing hyphens for usage of case sensitive (Not related to Sendfile but I noticed it when I was refactoring).

Changes introduced

• [x] Documentation Update: Fix typos and clarify SendFile usage.

Type of change

• [x] Documentation update (changes to documentation)

Checklist

• [x] Updated the documentation in the /docs/ directory for Fiber's documentation.