There appears to be some confusion with this feature, the default viewer and editor is used when there is no windows file association or internal file association to edit or open a file.
To override the windows file association for a file type you need to define a custom file association within FlashFXP (Options > File Associations)
For example to open php type files with a custom program you would create a new association entry, then add
*.php as the file mask and then set Open with to your custom application and then check the following options that apply and click the save button.
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