Seems like a search or filter field, like the bookmark manager area, might work instead. And it'd be more generally useful than specifically making tabs for different file types.
Is there a reason you don't just search in the operating system? Like Windows or Linux or whatever? They tend to have good search mechanisms already. The only thing missing would be which site the images came from.
I guess "Have Firefox add a 'downloaded from' or 'source' value to files" is another way to solve this. Many file types can have extra information hidden away inside them. Like the location a photo was taken at, or subtitles for a video.
I think NTFS even supports attributes in files called "streams", but nothing I've seen seems to use them. It's a way to make sections inside a file without changing the original contents. Regular programs still load the old stuff, and stream aware programs can ask about separate areas it hopes to find.
If you are looking for one specific file extension such as mp4, you can use the little search box upper right above the Downloads list (Ctrl+J, or on Mac, Command+J). But currently there is no way to filter on multiple extensions, the way you can, say, in a File Open dialog.