FileMaker Server tries to be smart about only sending containers to the client when specifically requested.
For pictures (.jpg, .png, etc.), FileMaker Server sends a thumbnail that is slightly larger than the container field itself. You can set FileMaker Server to store thumbnails for best performance, or have server generate them on the fly.
For movies and pdfs where the container field is set to Interactive content, FileMaker Server sends one frame / one page until the user plays the video or starts to scroll through the pdf. If Play automatically is set, then the stream starts immediately.
For all other files, and movies and pdfs where the layout does not do interactive content, FileMaker Server sends an icon, thumbnail, or frame (depending on file type), and only delivers the complete file if requested via Export field contents or another means.
I hope this helps.