I think it depends on the user and the context.

If you are referring to the extension itself, such as:

  Then save the file with a. html extension  

Then, since you’re referring to the “. HTML Extension “. html” to avoid coding and display an ambiguous file type. Use “a dot html” to explain the problem. In this case, you can’t call it “a html extension” since that doesn’t define what the extension actually is.

If you’re referring to the file itself, such as so: I

  should be referring to an AVI file there.  

Use the form “an AVI” here.

If you refer to files with the extension, such as Files

  should have. There 
 should be. Again, if you refer to files with the extension, such as. What are the files that can be viewed on an Avi file?  

To use the word dot, omit all the words. After that, add the word dot. Ignoring the fullstop when writing file types (not extensions) would be how I’d solve this. Truely a matter of preference, understandable either way, but slightly different.