I'm not sure what you mean by "requiring access to external files" in the context of an HTML/js/css course. If you're using a procedural language like Python or C, you can provide students with a data file by uploading it into the question using the Support files section of the authoring form. The student's answer code can then open the file and read it with code like
infile = open('<name_of_support_file', 'r')
data = infile.read()
(depending on the language, of course).
In an HTML/js/css course, however, this style of file processing isn't usually possible and I'm not sure what type of 'external file' you wish to access. Could you give me an example of a simple question you would like to write that uses such an external file, please, and I'll see if I can come up with a solution?