Open custom file extension in my app.

Hello. I have the following question. I have a program that creates files with its own extension and reads them. When I select a file with my extension in the Files app, it automatically opens my app. But if a file with my extension is attached to an email, when I click on my file it shows me a screen with programs to open (actually it shows my program the second time I click on the file, if I click the first time, my program is not listed) . I have a question, is it possible to make the file with my extension attached to the letter immediately open in my program without additional screens or is it not allowed. If possible, then what did I miss, based on the fact that it opens immediately from the Files program. Thank you.

P.S. File how it works below: ![]

[Image Edited by Moderator to Remove Personal Information]

Open custom file extension in my app.
 
 
Q