Handle extensionless files by MIME type (Dockerfile, Makefile, .gitignore)

[haiphucnguyen]

Currently, files without an extension fall through to application/octet-stream and throw UnsupportedOperationException. Fix:

• Detect by filename pattern (case-insensitive Dockerfile, Makefile, .gitignore, .env.*)
• Read as plain text when Tika detects text/*

Scope: FileContentExtractor.kt + FileTypeSupport.kt + unit test.