A little theory
The mapp View server generally delivers the contents of its web root directory to the client. The mapp View server is also able to deliver external content by using the file device functionality. More info in AS Help as well as in this Community post
MV6.3.0 introduced new security feature - File Device Whitelist
The Accessing external resources via file devices) is prevented by default. So after project update to this version of MV and newer it could be that your external files are not loaded with error 403 : Unexpected server response or similar (PDFWidget, VideoPlayer widget,..)
Key takeaway
- File Device whitelist configuration is accessible in advance section of mappView configuration
As usual, hoping that it helps 
Jaroslav 