Use .tsv instead of .tab for the tab-delimited download option

Currently, when people choose the tab-delimited download option...

... they get a .tab file like this:

This causes problems because .tab is not well supported as a format for tab-delimited data. Here's an example of someone who was unable to open a .tab file:

Fast forward to 2026 and Excel (for example) still can't open a .tab file. It's greyed out, like this:

The solution to to switch from .tab to .tsv, which is much more standard.

This will be a backward-incompatible change, but I think it's worth it.

If we simply make the change, I'd size this at a 20. I'm uncertain of how many places in the code we'd need to touch. Maybe there's a potential shortcut (hack?) where internally we keep the .tab extension but when it's presented for download we switch it to .tsv? What matters most is the experience of the person downloading the file.

If we add configuration for installations to opt out of the change, that is, to stay with .tab, it would increase the complexity. Give it a 30.

Another option would be to have a config option that says to leave all existing files alone (keep them at .tab) and only make new files .tsv files. Maybe this would be a 40.

We could wait until JSF is gone. That would reduce the amount of code we have to change.

Notes:

• https://en.wikipedia.org/wiki/Tab-separated_values lists .tsv before .tab under "file extenstions"
• https://fileinfo.com/extension/tsv says, "On occasion, TSV files may be saved with the .TAB extension."

Start at the tab-delimited download option and trace how its filename extension is generated and exposed to downloads. Search for all .tab references and related tests or configuration, then determine the affected paths and compatibility behavior. Done means the download presents a .tsv filename consistently, with the chosen behavior covered by tests.