Affected versions
Nightly 81.0b5

Affected platforms
Platforms: Windows, Ubuntu, Mac OS

Preconditions
Reach about:config and set the following pref: browser.download.viewableInternally.enabledTypes = SVG,XML,PDF,WEBP,AVIF

Steps to reproduce

1- Launch the Firefox Browser and reach the Applications panel from the about:preferences
2- For PDF, SVG, XML and WEBP file types displayed in the Applications panel, override
"Open in Firefox" option and change it to Save File from dropdown menu.
3- Reach https://mime.ty.ax/.
4- From the Prebuilt section click each File type PDF, SVG, XML, and WebP

Expected Result:
Each file is saved automatically and can be found in the downloads panel from the toolbar.
None of the files will open in Firefox automatically.

Actual Results:
Files with no content-disposition, default content-type, no extension are not saved automatically, but opened instead except for PDF link

Severity suggestion
I think the severity for this issue should be an S3 or 2 since the preferences set are not being taken into account.

This is expected behavior, sorry for confusing the issue when we were discussing it on Slack. The preferred behavior for file types doesn't normally apply to types that are supported in the browser, unless "content-disposition" is "attachment", which forces saving the file (and optionally prompting about how to view the file). The test plan for changing the preferred action shouldn't have included these links, that was my oversight when I added them to the test page.