🐛 Fixing bug in how we find a derived file

[jeremyf]

Prior to this commit, the requesting the thumbnail for the file basename "1234.ARCHIVAL.pdf" would result in a basename of "1234.ARCHIVAL.pdf.thumbnail.jpeg".

With this commit, we now return the basename of "1234.ARCHIVAL.thumbnail.jpeg".

Related to:

• https://github.com/scientist-softserv/derivative_rodeo/issues/56