Android: how do I intercept when the user downloads a file (from the browser or from the email or form other source)

Android: how do I intercept when the user downloads a file (from the
browser or from the email or form other source)

Hi and thank you for your help,
As posted in a previous question, that possibly is pointing me in the
right direction,
I need to intercept the event of the user downloading a file (from the
browser, form the email..) so that I can propmpt the user and save the
file in a folder defined in my app.
And, viceversa I need to be able to intercept the event of the user
uploading a file into a webpage, so that I can offer him to select the
file from the folder defined in my app.
If somebody could point me in the right direction I would really be
grateful!!!
If there is any open source solution already available that would be very
useful as well.
Thank you very much for any help.
LISA