First of all, yEnc is an encoding and not a file format. Once it is decoded it can store different file formats.
File format on the other hand is a specific format of a certain file that can be recognized by specific programs.
For example if it is an image file like JPEG or GIF - this can be recognized by many image viewers including the ones integrated in the Windows. Also news readers may recognize them as well and display them instantly in the browser window. However if it is a file format that Outlook Express or Windows Mail (or any different news reader) cannot recognize, then it will usually be displayed as a "paperclip attachment".
With all this in mind here are some common file formats you can find on the newsgroups.
yEnc, MIME, UU - encoding formats
yEnc is not a file but an encoding format. Encoding means that a file that is stored within is converted from one format (8-bit binary) to another (yEnc). yEnc can store any file format inside but it is not a file format itself but an encoding format. Other common examples of encoding formats on the Usenet are MIME and UU.
Following are various file formats that can be stored in yEnc (or MIME and UU) encoding.
JPEG, JPG, GIF, PNG, TIF, BMP - image files
Most of these formats will be recognized directly by your news reader. Once they are decoded from yEnc they will be displayed directly in the browser window. Then you may choose if you want to save them or to do whatever you wish with them. Also they will be available as paperclip attachment for saving.
AVI, WMV, MPG, MPEG, QT, RM - video files
Those are all video formats - Quicktime, Real Media, Windows Media Video etc. You need appropriate player to play these. Many (but not all) of the formats can be handled by Windows Media Player that comes with the Windows XP and Windows Vista. Some of these like Real Media and Quicktime may require specialized players. Others like AVI may not be playable in video player simply because you lack codec to view them. Codec is like a driver but for video format - it is required to play the file and must be installed on the system.
Some of the common codecs used are DIVX and XVID. You need to install those on your system before your video player may recognize those formats. Once installed, many players (and even Windows Media Player) will be able to recognize and play the video file correctly.
Also look for codec packs. There are several one of the well know ones is K-Lite codec pack. Once you install it - you will be able to play a wide variety of formats that AVI files can be encoded to. Also closely associated with K-Lite codec pack are Quicktime and Real alternative. Those are replacement codecs for QT and RM files which don't require you to have Apple or Real player but you may use any player you like because standalone players are compatible with all codecs that are installed on the system. There is also a good and very easy to use player called Windows Media Player Classic - comes with K-lite codec pack. Much easier to use than IMHO bloated Windows Media Player.
Of course you need to find a player that suits your needs but is also compatible with all the common video formats out there.
There is no need for many codecs on the system. Most common ones are DIVX (AVI), XVID (also AVI), QT and RM. Others are just bonuses but usually rare.
MP3, WMA, RA, MPA - audio files
Audio formats contain music, speech or various audio data. Meda Player Classic can play some of these formats. WMA requires Windows Media Player. There are some players that handle multiple formats. MP3 and WMA are the most common ones and both can be played in WinAmp.
ZIP, RAR, ARJ, ACE, 7Z, TAR, TGZ, LHA - archive files
The most common archive formats you will find are ZIP and RAR. You may find some of the other ones here and there but with those 2 you covered most of the ground. To unpack them you need a program that handles that archive format. Many tools handle multiple archive formats. Look for these tools on our links page.
Archives contain one or several files inside. They are called archives because they are packed and contain different file types inside - it can be programs, pictures, videos or any other.
PAR, PAR2 - parity files
Simply put, these files are used to recover missing parts of multipart posts. They have no purpose by themselves and you cannot read them with some reader program. They are only useful if you miss some part of some very important multipart post that you don't want to miss. In most cases you will not need PAR files at all and you can ignore them but in those few cases where they are important please familiarize yourself by reading the articles section - "what are PAR files" article.
Other - misc
I get a lot of questions like that yEnc file is not decoded when in fact it is - common reason can be multiparts in combination with missing data.
How do you recognize this?
If you see a post starting with =ybegin
- those are yEnc encoded files. They haven't been decoded from yEnc. You need software like yDecode to do this if you want to view them.
An example (beginning) of non-decoded yEnc formatted message:
=ybegin part=4 line=128 size=346200990 name=My Video.avi
=ypart begin=1536001 end=2048000
An example (beginning) of decoded yEnc formatted message (but missing some previous part or for some reason not being displayed as "attachment"):
Notice the difference? The first example begins with =ybegin, clearly indicating it is still in yEnc format. The other one starts right away but is obviously missing a part of data because it starts like that. This could be a part of a multipart file not properly decoded.
If they start with various data but there is no =ybegin
in the first line, they are decoded but they miss some essential part like previous part. They could be for example a part of multipart data.