As noted above, the compiler will take direct cognizance of files ending in .html, .mic, and .fs (case-insensitively). Now the purpose of the latter two is clear and distinct. These files should (though, they are not required) to contain the definition for a single fieldspace.3.2
The difference between the code within the other two types, .mic and .html, is not so clear cut. Both types will be compiled into viewable pages, and in this respect, are the same as far as the compiler and driver are concerned. Stylistically, however, most pages should probably be of the .html type.
The .mic extensions should be used consistently within a site. Beyond that, they should, if nothing else, indicate ancillary pages that are mostly perl code. For instance, a page whose purpose is to look at state and direct users to another page (without accepting any user input or doing much output) could qualify for a .mic extension.