owl

Rewrote handling of attachments:

- New file types are supported: ZIP archives, JPEG and PNG.

- Types are automatically detected using python-magic, known extensions
  are preserved. Types are stored in the database.

- Attachment size limit (which BTW did not work) made configurable.

- UTF-8 detection rewritten to be scalable.

- Links to download attachments now contain cryptograpically signed
  post ID, user ID, and recommended filename (for student's submissions,
  it's based on the student's name).