Export: Better detection of images that can be inlined.
Two new variables, `org-export-html-inline-image-extensions' and
`org-export-latex-inline-image-extensions' now define the file
extensions that will be considered for inlining images. The default
for HTML is png, jpg, jpeg, and gif files. The default for LaTeX is
png, jpg, jpeg, and pdf, assuming processing with `pdflatex'.
For LaTeX export, this can lead to conflicts for links that point
to non-image PDF files. To avoid that such a link will be inlined by
accident, always give a description text for such a link, one that is
not equal to the path.