I find that results can be a bit inconsistent when typing "jpg" into the search bar, but are accurate when I type "jpeg". Have you tried using the faceted search option instead (make sure you've selected All Photos as the source, then click in the empty search bar to produce the list of searchable facets, click on extension to produce the list of file extensions, select the one you want). The only problem with using the faceted search is that .jpg and .jpeg are listed separately (if you have file extensions of both types in your library), and the search results are restricted to the actual extension that you select....so to see all jpeg/jpg files it's necessary to do two searches. However, when I type "jpeg" into the search bar the results do include both .jpg and .jpeg files, but typing "jpg" excludes .jpeg files and doesn't seem to pick up all the .jpg files. That latter issue feels like a bug to me.