File uri MIME type check

The file test.gif has some javascript appended to the end of the file.

It is executed when loaded as a script:

<script src="test.gif"></script>

1. Download folder locally and open testload.html in a browser.
2. If you get a javascript alert, your browser did not enforce MIME type check before executing the .gif

Check console for error message.

Chrome:
Refused to execute script from '.../test.gif' because its MIME type ('image/gif') is not executable.

Firefox:
Loading script from file: URI (“.../test.gif”) was blocked because its MIME type (“image/gif”) is not a valid JavaScript MIME type.

To fix on Firefox:

1. goto about:config
2. set property security.block_fileuri_script_with_wrong_mime to true