Description

Fixes #19268:

Less files can contain @plugin "plugin.js"; style imports, which reference JavaScript files. When Vite runs on a .less file that contains a @plugin import the makeLessWorker function calls viteLessResolve with the plugin.js file (in this example), which then calls rebaseUrls, which is expecting to have received a CSS file and runs a bunch of regex including rebasing url(...)s, which is breaking my JavaScript file which contains that string!

This PR resolves the issue by using the mime value that comes from less and indicates that it's loading a JavaScript file. For .less files this mime value is undefined, so checking explicitly for application/javascript seems like a good option. There is no doubt another option for detecting this situation (when we shouldn't treat the file as CSS and rebase it).

I have added a test case based on the example...

Example

I have setup a basic example of this bug at https://github.com/karlvr/vite-less-plugin-bug:

• The src/main.less file imports the src/plugins/test.js plugin and then uses it in the background-image attribute.
• The src/plugins/test.js includes the string url( to demonstrate the bug, as rebaseUrls mistakenly transforms it, thinking the file is CSS
• The output dist/assets/index-CWZCrLGA.css demonstrates the output; the url(...), which should just contain the data uri, is prefixed with the base of the src/plugins/test.js file: url(plugins/data:image/png;base64...