Video File Extensions

Please take a moment to offer your preference.

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I am writing a script that I want the users to be able to upload an audio and/or a video file if they desire. I chose to allow an mp3 file extension for the audio, as I liked the idea of its compact size. I researched the list of popular video file extensions, but I could use a little feedback. Please keep in mind that this script could possibly service thousands of users.

1. What file extension should I allow for the video upload? I want it to be popular, yet economical as to the amount of space, resources, etc.

2. What type of video file extensions do cellular phones use? Can they be easily uploaded as a video clip? I know nothing about cellular phones!

3. How many kb should I allow for the audio upload (thousands of users may be uploading files - preferably 4 or 5 sentences long)?

4. How many kb should I allow for the video upload (thousands of users may be uploading files - preferably 4 or 5 sentences long)?

Thanks for taking the time to offer your opinion!