[dev] default thumbnail displays for PDF files

[rjkati]

Descriptive summary

On the work show page, the default thumbnail displays rather than a preview thumbnail of the PDF file.

Image thumbnails in the UV and mp4 do display as expected.

Expected behavior

PDF thumbnails should display above the download link and in the item view list on the work show page.

Actual behavior

The default thumbnail image displays for PDFs, rather than a preview thumbnail of the PDF file.

Steps to reproduce the behavior

Example: https://dev.nurax.samvera.org/concern/generic_works/rr171x51k?locale=en

Related Work

samvera/hyrax#4788 -> addresses a similar problem with the Monograph work type, but the issue described above is occurring regardless of the work type selected.