Support for downloading base64 files

[flibbles]

The blob doesn't work for downloading a tiddler type that calls itself out as a base64 type. Using data urls does work.

This change makes it possible to download png and other base64 encoded tiddlers correctly.

It's for TW5-Graph, but it struck me as rather amazing that there does not appear to be any way to download an image tiddler as an actual image. Seems like something SOMEONE would have wanted to do by now.

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📊 Build Size Comparison: empty.html

Branch	Size
Base (master)	2418.3 KB
PR	2418.6 KB

Diff: ⬆️ Increase: +0.3 KB

[flibbles]

Without this, it seems to be impossible to download an image/png tiddler as an actual png tiddler unless one makes their own javascript solution that does not touch the saver-handler.

With this, you can flag an image/png tiddler as image/png;base64 and use the plaintext template, and it'll download just fine.

[flibbles]

Added a comment to explain to future coders why we'd use data urls over blobs for base64 tiddlers.

I would write tests for this, but I can't think of a way to do it that wouldn't involve a big infrastructure of mocked system methods, and would be rather arbitrary anyway. Since you can only know which download style you want from seeing how they react in the browser.

[flibbles]

I'm fine if this gets rejected, by the way. But then I would say that TW should include $action support for downloading images. tm-download-file should be able to download images without needing to specify that they're base64 encoded (like this PR requires).