Renaming a file does nothing to change its contents. A Flash Video (FLV) file is not at all like an Audio Video Interleave (AVI) file.
There are two explanations for Windows suddenly accepting a file that you have merely renamed.
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Firstly, it was an AVI file to start with, and you have merely changed the extension back to what it was supposed to be.
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The second explanation is that Windows is very silly about file name extensions. It expects them to define the contents.
If you have lost the association between the extension ".flv" and any program that might be able to display that format, then you will find it fairly hard to get any program to look at it.
By renaming it to have an extension that Media Player, for instance, will at least look at, it means that you get to open it. Media Player then looks at the CONTENTS of the file, recognises that it is really Flash Video, and displays it successfully.
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Now, the second explanation also might explain the first explanation...
If someone had trouble viewing an AVI file, because the file association was broken, they may have renamed the file with .flv, because they knew that did work.
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