Dan
I wrote the message below before I saw your latest reply, which indicates you know what you are doing. But I'm still not clear what your aim is beyond normal external "download" file opening?
I'm sure we can lick this.
Best wishes
Josiah
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Ciao Dan
This is not per se a TiddlyWiki issue. Opening external files depends on the operating system as well as the browser. And, as Mat mentions, its less easy than it once was. BUT, for desktop browsers, at least, or at least Firefox, you should be able to do it. What actually is happening is that
the browser, if it does not recognise a file type it can deal with natively, passes the file to the operating system via "download" / "save". Then, depending on how you have the OS setup, and also how individual apps work, the file can directly open in the app.
The mechanism for Firefox is described here...
http://kb.mozillazine.org/File_types_and_download_actions... it shows how to add new types as well as to make the process of "download-file-opening" automatic.
Note, the file opened will usually NOT be the original file, but a copy.
A way to establish whether Firefox on a Windows desktop will open file types is to enter ...
file:///C:/... into the address bar, navigate to a file you want to test, click on it and see what happens.
Just to be sure, I tested a TW Tiddler with a link to local files, exactly like you, with filetypes not supported by the browser. It worked fine for me.
A similar mechanism used to work in Chrome too.
If you can't get it working ask again with a bit more detail of what happens, your OS & browser.
Best wishes
Josiah