How do I transfer files from Android cellphone

[Bret Busby]

Hello.

I have an Android cellphone that I connect to my UbuntuMATE 18.04
computer via USB tethering, and, use that for accessing the WWW on
that computer.

On my cellphone, I have a number of image files (screenshots) that I
want to transfer to the computer.

Rather than attaching each file to a gmail email message composed on
the cellphone, and, sending a copy of the file, for downloading to the
computer, how do I direcly transfer the files?

I have searched using google, and, the closest that I found to an
answer, states that, upon connecting an Android cellphone to a
computer, via USB, a window automatically pops up on the cellphone,
with an option for MTP file transfer.  This does not happen with my
cellphone.

The cellphone does not show as a storage device in the caja file
manager, for cut/copy and paste file transfers.

So, how do I transfer files from my cellphone to my computer, when the
cellphone is connected to the computer via USB?

Thank you in anticipation.

-- 
Bret Busby
Armadale
West Australia
..............

"So once you do know what the question actually is,
 you'll know what the answer means."
- Deep Thought,
 Chapter 28 of Book 1 of
 "The Hitchhiker's Guide to the Galaxy:
 A Trilogy In Four Parts",
 written by Douglas Adams,
 published by Pan Books, 1992

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