Unclear how to find more granularity of files beyond "File Type" (application, tabulardata, data, etc.)

If you got to https://dataverse.harvard.edu/dataverse/polbehavior and click "Files" you see "File Type" as in the screenshot below, but the detail is not very granular. You see files flagged as application, tabulardata, data, etc.:

What if you want a specific type of file such as an R file? It's possible but you have to know which undiscoverable field to search on, which in this case is fileContentType. Specifically, a search for fileContentType:type/x-r-syntax ( https://dataverse.harvard.edu/dataverse/harvard?q=fileContentType%3Atype%2Fx-r-syntax ) will show 1,553 as of this writing in the Harvard Dataverse:

fileContentType (and related fields, possibly) should be exposed on the Advanced Search page, which currently looks like this:

The way I figured out to look for type/x-r-syntax is by looking at a list of files for a dataset in JSON format with this: https://dataverse.harvard.edu/api/datasets/:persistentId?persistentId=doi:10.7910/DVN/SCN9LA

The background on this issue is that this morning @christophergandrud were discussing how one would identify all the R files in an installation of Dataverse. As a starting point, we looked at http://dx.doi.org/10.7910/DVN/SCN9LA (screenshot below) which I happened to know has an R file called "Thal PB Replication.R" that can be used to reproduce plots in a paper. (Once this platform for reproducibility has been publicly announced, I'll mention it. 😄 )

Anyway, with fileContentType:type/x-r-syntax as a starting point, one should be able to iterate through all these R files via Search API: http://guides.dataverse.org/en/4.6/api/search.html#iteration

(This issue is related: What are the allowed search fields for the Search API q parameter? #2558).

Start with the Advanced Search page and the Search API field discussion in issue #2558. Trace how searchable file metadata is represented, especially fileContentType and related fields, then expose the appropriate fields for advanced searches and verify that users can find specific file types such as R files.