One thing you might find useful is to use PDB (the Python Debugger), in tandem with print statements like this.
You can make PDB break in your program by putting in:

import pdb; pdb.set_trace()


where you want the break to occur. Then, execute the steps to hit that line of code, and switch to your server window - you'll notice that the server has become a debugger. You can then next (n) through, continue (c) and print (p) things.

Unfortunately, Review Board never really has the files on the OS disk to read. The depot_filename that you've called this function with is the name of the file in the repository. To get the contents of this file, you would need to use get_patched_file.
However, we do not want to do that just to see if the file is modified as this function is called by endpoints just to retrieve metadata about files in a diff.
The JavaScript reposonsible for loading file contents for display in the DiffViewer is RB.DiffViewerPageView (in reviewboard/static/rb/js/pages/views/diffViewerPageView.es6.js). Specifically in queueLoadDiff, which calls RB.DiffReviewableModel.getRenderedDiff (in reviewboard/static/rb/js/diffviewer/models/diffReviewableModel.es6.js).
The Django view for actually rendering the content (that the above JS model calls into) is reviewboard.diffviewer.views.DiffFragmentView (in reviewboard/diffviewer/views.py). This is where we will want to do the check call this function with (1) the filename and (2) the old/new file contents. Then we can take into account similar logic to  how the contents of deleted files are requested to show the contents of minified files.
I realize this is a lot to take in, but I'm happy to answer any questions (esp tomorrow in person) about this. I've spent quite a lot of time in the DiffViewer's JS recently, so I'm fairly familiar with how it all sort of fits together.

I find it easy to do import pdb; pdb.set_trace() wherever it is im using pdb. That way I don't forget to remove imports :)

Nit: undo all these removed blank lines.

We don't do multi-line comments with docstrings. Instead we do multiple single-line comments with #.

Nit: blank line between statement and block.

Undo.

This will need to be a new class so we can use a separate event handler.

Please add a screenshot as a file attachment.

Can you detail what testing you've done?

"minified_bool" isn't a good variable name (I can infer the type from the assignment), plus we only use it once. Let's just do it inline:

f = {
    'minified': 'min' in depot_filename.split('.'),
    ...
}

Can we add a comment explaining what the heuristics are?

This blank line isn't necessary.

Can you add a doc comment for this?

Could use some rewording for clarity, like "Checks the number of lines in the chunk to determine if they are short and not already categorized as minified files." (Based on your code it seems that you mean the number of lines in the chunk, not the length of the lines in the chunk so clarifying this in your comment would be helpful.)
Could be more clear on what "short" means (give a number)? It seems that you are saying a file is minified if the number of lines is less than 20?

Tiny nit: two spaces in front of trans should probably be one.
Also this line should be indented to be nested within the <td> element.

Should there be an additional linebreak here to separate the two paragraphs?

This comment is a bit unclear, particularly the second part of it. I think the commenting might end up being unnecessary if this nested structure can be cleaned up to be more immediately readable.

I believe this should have an extra line break since the docstring is on a single line.

Nitpick: I had to re-read this comment a few times to try and make sense of what it was saying. Could you try to make it a bit more clear? Maybe something like:
"It checks the file's chunk data to determine if it is possibly a minified file or not. If there are less than 20 lines, and more than 500 characters in a line, it is determined there is a minified script in the file."
Some thing like that? Correct my logic if it's wrong haha