go server "multipart/form-data" works well for closed networks, but not for online networks.

While studying the Go language, I encountered an error in the most basic file transfer. I came here to leave a question because I couldn’t figure out the cause.

I have implemented a webui server that can be accessed through a web browser. This server is running on my local PC and accessed using Chrome/Edge from an external PC. In this state, a file transfer was attempted using multipart/form-data, but it was confirmed that the file was not read and was transmitted as an empty value.
After various attempts to resolve this issue, it turned out that the files worked fine offline, but the files could not be transferred online. (Windows 10: Chrome, Edge)
I thought it was a simple Cors problem. Even using ‎FileReader.readAsDataURL() works only for the same offline closed network.
If a FileReader error occurs, the error message is:
DOMException: The requested file could not be read, typically due to permission problems that have occurred after a reference to a file was acquired.

I confirmed that it works normally even in Chrome v110 version. Has anyone else had a similar problem?