Use streaming for UploadFileReplace

[piotrbulinski]

This pull request refactors the UploadFileReplace method in files.go to use a streaming approach for constructing and sending multipart form data, rather than buffering the entire file in memory. This change improves memory efficiency, especially for large files.

File upload streaming improvements:

• Switched from buffering the entire multipart request in memory using bytes.Buffer to streaming the multipart data via an io.Pipe, reducing memory usage for large file uploads.
• The multipart form creation and file reading are now performed in a goroutine, writing directly to the pipe, which allows the HTTP request to read the data as it is being generated.

Minor cleanup:

• Removed the unused import of bytes from files.go.

[piotrbulinski]

@jonhenrik13 once #86 is merged, this branch can be updated and then it's ready for review as well.