Acceptable File Types
If I give you some data which is stored in an encumbered file format, you are giving a limited monopoly to the company that owns that encumbered file format. For example, if I give you a sample of me playing guitar, and its in mp3 format, then it is encumbered. The mp3 file type is not free, there cannot be a free (as in freedom), mp3 player for your computer, because all mp3 players are forced to give money to the owner of the mp3 file format. This is strictly true for both hardware mp3 players that you can put in your pocket, and also software mp3 players on your computer.
This is a bad state of affairs, so here's a list of common, encumbered file types, and my views about them.
Audio / Video
mp3
There are good open alternatives, and so ideally, I'd like to never touch mp3.# Alas, it is so popular, that other people sometimes require it.
mpeg
I believe it is in a similar position as mp3, encumbered, and very popular. mpeg is used within DVDs, which also have a pain in the ass (and broken) encryption system.
wma, wmv
These are Microsoft's audio and video formats. Its bad enough being encumbered, but Microsoft haven't attempted to make their codecs cross platform. Therefore, I never want to touch these.
Office Documents
doc
This is the encumbered format for Microsoft Word. There is now an open standard for text documents, and so nobody should continue to use the "doc" file type.
odt
Also known as Open Document Text This is a truly open document format, and should be encourage.
docx
Also known as ooxml ( Open Office XML ). Microsoft's new, botched document format. Its not open (so even the name is a lie), and its a pile of shit. Never ever use it, there really is no need. If you want a 20th century format, use "doc", and if you want a modern format, use the Open Document format.