refactor(files): simplify file filtering and default to text

[deathbeam]

• Remove filetype-based filtering from file list, now only filters out
  empty entries.
• Add simple binary detection in get_file by rejecting files with null
  bytes.
• filetype() now defaults to 'text' if detection fails, letting content
  validation handle unreadable files.

This improves robustness and performance by avoiding unnecessary
filetype checks and handling binary files more gracefully.

Closes #1533