Either from a computer or from a mobile, a web form with <input type="file"> allows to pick a file, or a picture from the smartphone camera.
Is it possible to specify a label that would show up:
?
For instance, for a face-recognition application, the label could be "Take a picture of a friend's face", to make it clear what the user is expected to do.
I would like to do this either from the HTML code or from a JavaScript script.
You cannot do this. The subtitle of the file selection dialog is the same in all cases, at the camera (at the most mobile operation system) also constant.
But pimarly it is a system specific question.