We're deploying a Shiny dashboard using flexdashboard, authoring on R 3.5.1 and RStudio 1.x on Windows 10 and deploying to Shiny Server Pro 1.5.7.x on RHEL.
We're using includeHTML to load an HTML file that in turn loads an inline image located in the same directory as both the Rmd and HTML files. That HTML file renders fine in the RStudio browser and when launching the app in desktop browsers. On Shiny Server, however, the image fails to load. This is only a problem for local image files - loading remote images is fine (makes sense, as it's the browser doing that loading). It doesn't look like a permissions issue, as chmodding them liberally doesn't yield anything.
I've seen references to this problem in the context of ShinyDashboard, and am aware that there are some prohibitions on serving arbitrary files. I don't see a solution in the context of flexdashboard, however, nor an option to modify or disable a setting in Shiny Server config.