after reading again you requirements
your only posibility is to use the activity starter solution (or @TIMAI2 's extension, which is based on the activity starter) to open the client mail app to send the email from there
To avoid picking the file after the image has been copied to shared storage use a file extension to convert the filename to a content uri... my file extension offers the GetUri method for that
Taifun
Okay, good thing you noticed. I was telling you again, that I didn't think your proposal was fair.
I will then evaluate this last extension.
Thank you
???
The question is, why do you have that requirement?
You could have an app specific Gmail account and all users of your app send emails using that account without even knowing user and password of that account. .
Taifun
Because I'm a little crazy and also stupid 
I am joking. Mainly because I want the user to manage it with the best existing email client (and in any case with the one he/she prefers), not with a limited client created by me.
I want the message to not be sent automatically after the shot, but to optionally add anything else after.
And I thought that with MIT you could send a photo you just took to an email client in a more native way, also being able to set pre-filled fields.