I don’t know exactly about this forum (without trying it), but other forums that have limits on uploads are simply checking the file extension text.
So, the way I’ve used to bypass this situation is to add the extension “.zip” after the file’s normal extension (simply changing the upload’s extension to one that is acceptable.) In the same post I say what the real extension should be and how to delete the extraneous “.zip” after the download.
In your case, you could add the extension “.zip” to “LightburnFile.lbrn” to upload “LightburnFile.lbrn.zip” and all should work after explaining to delete the “.zip” at the end of the file name after downloaded.
PS. Or you could do just as Will says and zip it up before download.