I went through what you posted. Changed the mp4 type to 'open download dialog' now when I click on it (not right click) it opens the dialog which saves the file in the correct format

Go back to 12.17. Enter opera:config in the address bar, click on the user prefs tag and enter this into the Custom User-Agent field

Mozilla/5.0 (Windows NT 6.1; rv:31.0) Gecko/20100101 Firefox/31.0

It will solve a lot of rendering problems that browser sniffers cause. Not all the problems, but enough of them to allow us to keep using opera presto, for a while at least.

I have already said the problem is at my end and that it is all pages and just mp4. Anyway I have figured it out. I am using the extension Imagus by Deathamns, when I disable it everything behaves as it should.

So it works for you. How does that help me?

When I copy the link it is an mp4, when I click "save as" it is as an ogg, and it is converted, renaming it does not work.
It is all mp4 files not just a particular site. There are three test files on this link, hover over each one and it gives the correct extension, click 'save as' and it is an ogg. Except the ogv, that one is saved correctly.

https://archive.org/details/Pbtestfilemp4videotestmp4

opera 12.17 64 bit.
When I right click on an mp4 link and "save as" opera is saving it as an ogg file, which will not play as a video.
The 32 bit version works fine so I assume it is a settings difference.
Looking in my preferences->advanced->downloads mp4 should be saved as mp4. So I have no idea why it isn't.
Any advice?

And it is getting worse; No open image, no image zoom, and speaking as a developer no editing of page source. Good job 12 still works because that is now the only browser that lets me edit the source.

Change for the sake of change, not a good thing. Selling your soul, even worse.

I have just upgraded from 12 to 22 and it is time to part company. Here are the reasons why;

Bookmarks: The properties window has gone. For rarely visited websites I used to keep useful information in here such as login names. And I want a bookmarks drop down, not a bar.

Links: I cannot turn off the underline. I have no idea why but underlined links annoy the hell out of me.

Image in page: The icons for 'fit to width' and 'turn off images' were incredibly useful. Now they don't exist.

And the deal breaker,
Context menu: The option to open in 'this' tab has gone. This is the one thing that set opera apart from all other browsers, I could decide which tab to use. Google is set to open links in a new tab, however there are occasions when I have too many tabs open and I do not want to add to the mayhem. After a quick search I could open it 'here', though not any more.

I knew that opera was using chrome as a backend, but opera now is just an uglier version of chrome.