If you save any file to your computer, then it will be a new file on your computer and so the computer will set the 'Date Created' to the date it was saved as a new filw on your computer.

You can see various information in the saved document Properties. An image usually also has the 'Date taken'. You can also choose to display this info in the column headers. (not in Thunderbird - I'm talking about where it is saved on your computer) So if saved in eg: Documents Use the 'View' option 'Details' then you can right click on the column headers to see optional column headers, click on 'more' to reveal a greater number of options. There may be a 'Content Created' option.

Many documents will contain a date when the person compiled the document. You could rename the file to include that date for easy reference. But there is no method of Thunderbird knowing when someone created a document. The email will only contain the date the email was sent or date you received it.

Thank you so much, Toad-Hall, this is very helpful! It's like I expected, but I hoped there was some little trick I didn't know of. Thanks for your explanation!