The team is not currently considering providing an extension option. Still, I've understood you are looking for a way to decrypt an encoded request or response body from what I've understood. If that is correct, this is a functionality already present in Fiddler Everywhere for many known encodings (like gzip, deflate, etc.). The decoding is enabled by default for all inspectors except the Raw (where you could manually decode - refer to this documentation article for more details on how to decode in the Raw inspector).
Please do let me know if you are experiencing other issue or if you want to decode a specific encoding which is not yet supported in Fiddler Everywhere.
Thank you for your feedback! I can confirm that at this moment, the feature to work with external DLLs is still not supported in Fiddler Everywhere. I am posting an automatic English translation of your issue description and marking it as a feature request to be researched further by the team.
I don’t mean the problem of decoding. I mean that the request body is encrypted, such as AES encryption. The request body is a string of cipher text. In the past, Fiddler could use the extension DLL to decrypt and display it by itself. In Fiddler->Inspectors, now FiddlerEverywhere hopes to also provide extended functions. This function is really practical and helpful to our software testers and can greatly improve work efficiency.: