If there's a newly-added minified filename, I think depot_filename will be None, which might crash. This should also use single quotes, and probably test for .min..

This comment should have an "Args" section explaining what e is.

Because this is ES6, we can use a slightly shorter syntax:

_toggleCollapseDiff(e) {

These should use const instead of var.

Should be indented only 1 space within its parent.

Don't know if this is helpful at all, or even a part of the requirements of your project, but if you did want to maintain the collapsed state of your elements even after page reload, you could save a list of collapsed file ids under an object in browser storage (probably local storage) indexed by the current review request id. If the list under the review request id exists, use it to determine which elements should be collapsed on client-side loading. If it doesn't, remove all other currently saved review request collapsed-lists (to not use up a bunch of local storage space).

Just a couple code formatting things I noticed.

This is really difficult to read. Since collapse_by_default is a boolean value, I think it makes more sense to assign it as a boolean instead of changing it's value in an if statement.
I'm thinking something like this:

        collapse_by_default = (self.diff_file['minified'] and
                               not self.diff_file['binary'] and
                               not self.diff_file['deleted'] and
                               (not self.diff_file['moved_or_copied'] or
                                self.diff_file['num_changes'] != 0) and
                               (not self.diff_file['new_file'] or
                                self.diff_file['num_chunks'] != 0))

Looking at the other if tags in the file, they all have a space before closing the tag: ...show_deleted %}