Somethings it will be necessary to convert media files to formats supported by the players at our publisher sites. It may also be necessary to retrieve properties such as bitrate and video dimensions from the file media file itself. For the latter purpose, use MediaInfo
For conversions, use for example: The freeware Free FLV Converter from Koyote Soft.
When converting, you find the specs of the original file with the MediaInfo program (text view shows everything needed to know). See the example image on the right.
How to use MediaInfo
Download and install MediaInfo (use the link above)
Press the little button in the top-right corner with three dots (...)
Select the file you wish to analyze
Select View -> Text
The recommendation is to convert all video media files to flash video (.flv), maintain the same framerate as the original file, but lower the bitrate (~2000 kbps is usually enough) to make the video file smaller. A reasonable aim is to have all video creatives less than 3 MB. If you lower the dimension of the clip, make sure the aspect ratio remains the same. VLC  is a useful player to verify the results with.
How to use the Free FLV Converter
Download and install the Free FLV Converter (use the link above).
Open FLV Converter and press on the "Conversion to FLV"-button.
Select the file you wish to convert.
A new window opens: Set the configuration according to the settings found with MediaInfo earlier (size, bit rate, aspect ratio, frame rate). You can also set the output folder (where the converted file will be saved).
By the default it saves to the Desktop, otherwise check the folder you specified in step 4
(Note: it also saves 3 other extra files which can be safely deleted, we are only interested in the flv-file.)