Flash Decompiler can convert Adobe Flex files.
For your convenience you can see whether it is a Flash or Flex file:
1. You’ll see 
or 
icon above Resource panel;
2. When you click 
button there will be a tooltip “Convert into FLA” or “Convert into Flex”;
3. “Convert into FLA” or “Convert into Flex” tooltip will be in Main menu → File:
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Remember that Flex files are created by Adobe Flash Builder, therefore they cannot be converted into FLA. Flash Decompiler will create a Flash Builder project, and you will need Adobe Flash Builder software to edit this project.