You can start by using BitziWeb-Lookup before downloading a file

Instuctions here:

Bitziweb

Also look at filesize. A MP3-file should be 2.5 MB and upwards. A decent quality movie at least 650 MB

Stay away from zip, exe, mov, wma, asx. A good idea would be adding extensions to your keywordfilter in:

Tools<Options<Filter<Keywords (remember the period when adding. For example .zip or .wma
Also add keywords for files that you don't want

Don't make searches too specific as it often will bring up bad results