1. .mp4
    The rarity of getting an email with a video attached is matched only by how special a video must be to send via email.
  2. .mp3
    Someone thinks enough about you that they heard a song you might like and sent it to you.
  3. .gif
    Never ever ever bad. Except how they slow everything down :(
  4. .zip
    The mystery is alluring.
  5. .png
    You're sending me a screenshot so it must be funny or useful.
  6. .jpeg
    Almost always something someone found on Google image search.
  7. .pdf
    Practically a guarantee that you're about to look at something pretty or thought provoking.
  8. .txt
    Rare, but so utilitarian it's cute.
  9. .xls
    Rank changes entirely based on how much you enjoy pivot tables.
  10. .csv
    This data set is either really useful or entirely unreadable. Enjoy!
  11. .doc
    "Ugh." -everyone opening a .doc, always.
  12. .ppt
    PowerPoint can go fuck itself.