File download updates

[jennifer-shehane]

Initially addressing docs for cypress-io/cypress#14431 but I think we can leave this open to see if more work will merge in for 6.3.0, to further update the download docs.

What to update?

• I added a new section to 'writing and organizing tests' explaining 'Asset files'.
• I wonder if the 'Screenshots and Videos' page should be repurposed to 'Asset Files' doc? I've always leaned against doing that because I felt people would search and understand screenshots/videos more easily.

Some other sections of docs that require updating:

• Change download directory: https://docs.cypress.io/api/plugins/browser-launch-api.html#Change-download-directory @chrisbreiding is this still relevant? Will this example work?
• FAQ about file download: https://docs.cypress.io/faq/questions/using-cypress-faq.html#Is-there-a-way-to-test-that-a-file-got-downloaded-I-want-to-test-that-a-button-click-triggers-a-download

[chrisbreiding]

Change download directory: docs.cypress.io/api/plugins/browser-launch-api.html#Change-download-directory @chrisbreiding is this still relevant? Will this example work?

Updated this to be about adding a unique mime type, as changing the downloads directory can now be done with the downloadsFolder option.

FAQ about file download: docs.cypress.io/faq/questions/using-cypress-faq.html#Is-there-a-way-to-test-that-a-file-got-downloaded-I-want-to-test-that-a-button-click-triggers-a-download

I'm not sure about this one. All the ways listed to test downloads are still relevant and valid. For example, a user might still want to test an href since that will be faster than downloading the file and they might not care about the contents.

If anything, maybe we can move up the part about downloading the file since it has better support now. We should still point users to the recipe, since it shows them how to then test the downloaded file itself.