feat: PARSER-64 add MDX and 5 image file types to FileType enum

[dimitrystd]

It is just for your visibility

Summary

• Added 6 new enum entries to FileType.java to sync with ContentFileType.java in the TMS parser model
• New types: MDX (text/plain), IMAGE_PNG, IMAGE_JPG, IMAGE_WEBP, IMAGE_GIF, IMAGE_BMP (all image/*)
• Entries appended after VSDM in the same order as ContentFileType.java

Test Plan

• FileTypeTest - 8 tests verify correct MIME types, isTextFormat flags, and name-based lookup (snake_case + camelCase) for all 6 new entries
• Full smartling-files-api module suite passes (37/37)