When you upload a video to the Media Folder there is a 100mb limit. Video files over 100mb require a data source to get the video into the app.
Video file names can contain no spaces. Video file names can contain only alphabetic and number characters. So for example you can't use "myfile$.mp4" because it contains the character "$".
When using "Access by Name" the filename has to be all lower case. So for example if the filename is "MyFile.mp4" then when using the filename in a Block it should be put as "myfile.mp4"