Add file extension identification config options

[nightlark]

Add config options for the file type id based on extension to:

• Recognize a list of additional extensions
• Ignore extensions that would typically be recognized

[KendallHarterAtWork]

How should the ignore option interact with the "include all" option? Should all files still be included and a warning issued or should it still exclude those files? I'm not sure which is more intuitive/useful.

[nightlark]

I feel like it would be more useful to have it still exclude those files, since then unknown file types can all be easily included while leaving out a specific set of file types that a user knows they don't care about getting info on (e.g. MS Word Docs, PowerPoints, etc).