There are many issues you will need to investigate. There are a number of docs on optimizing for web direct that you can web search for that will be very helpful. I'm going to (pun intentional) provide a thumbnail sketch of what they should tell you:
Less is more--keep your layouts very simple. Don't put large numbers of your images/videos on the layout at the same time. Use the smallest Thumbnails of your images and videos that you can and switch layouts to view the image/video one such at a time on a dedicated layout. FileMaker can be used to produce and store image thumbnails with a resolution that you specify.
Use themes in your layout design and do not add a lot of local styles to it. Save local styles into your theme in order to get faster results.
There's a lot more to this, but that's to get you started. (Sitting in an airport on my way home from FileMaker DevCon where I attended several sessions on optimization.)
You mention a 50GB file.
But of course - with external storage, the FileMaker file doesn't have to be anywhere near 50GB. All the stored assets can add up to 50GB (or whatever it grows to) but the FileMaker file itself will be quite modest in size.
But - perhaps you meant the FOLDER on your server holds 50GB of data.
Given that you are just starting to organize this, I guess that seems more likely.