Often operating systems know to ignore the extension of a file and read the actual metadata of the file. That may be why certain apps can see the video. They know the file extension (.fla) is incorrect.
It's common that people get the file extension to save wrong when exporting a video or publishing a flash animation.
If you published this from flash, rename what was published from .fla to .swf
If you exported a movie (file->export->export movie) then the extension could be many things based on what format you exported. If you remember it was quicktime, or you're on a mac, then try .mov or .mp4. Otherwise if you're on windows try .avi or .mp4. One of those will probably work.
You can sometimes get file details to help with this by right-clicking on the file and selecting properties (on windows) or file info (on mac) and it may read the header of the file and tell you what type it is.
If you're on windows, file extensions are hidden by default. If you open windows explorer (Windows Key + E), then in the upper left hit the Organize->Folder and Search Options->View Tab (at top), scroll down a smidge and uncheck the 8th option down so "Hide extensions for known file types" is UNchecked. Then you will see the files extension.
