Local-file I/O: problems with image paths containing dots

[pkgw]

I recently had a document where Tectonic was unable to include an image with a name of the form foo.bar.pdf. There seemed to be two problem at work:

1. If I just specified the filename as foo.bar, Tectonic would not be able to find the image under the name foo.bar.pdf. I think this is because it thinks the filename already has an extension and does not want to change it.
2. If I specified the filename with the .pdf extension, Tectonic still can't load it. I see that it is successfully opening the file and reading its contents, so something higher-level must be going wrong.