I've experienced this issue before when establishing a publisher backend for a D2D pc game business. It seems to be basically impossible without a Java applet of some kind, and even then it's wonky at best and just 'fails' at worst. The real fix for the issue seemed to be simply providing an FTP connection and letting people connect through the native client of their choosing.
That really seems to be the key for this problem, develop a simple native app capable of FTP uploads, that make it easy for users to deliver files to your app within the context of their use. Most browsers are capable of opening native applications via unique protocol, you could easily enrich the process by having the native app be a part of(or try to blend seamlessly with) major browsers.
That really seems to be the key for this problem, develop a simple native app capable of FTP uploads, that make it easy for users to deliver files to your app within the context of their use. Most browsers are capable of opening native applications via unique protocol, you could easily enrich the process by having the native app be a part of(or try to blend seamlessly with) major browsers.