OK, the results are below. I know I can write into the "attachments" folder as I also created a test2.php file in this same website, where I used DW rather than FB to create the file field and it works ok.
Status object: WA_UploadResult1
statusCode : 0
errorMessage : No file uploaded
clientFileName :
fileExtension :
serverFileName :
serverSimpleFileName :
serverDirectory :
contentType :
fileWasOverwritten :
fileWasSkipped :
fileWasRenamed :
fileSize : -1
isImage :
imageWidth : -1
imageHeight : -1
WA_UploadResult1_1:
clientFileName :
fileExtension :
serverFileName :
serverSimpleFileName :
serverDirectory :
contentType :
fileSize : -1
imageWidth : -1
imageHeight : -1
statusCode : 0
errorMessage : No file uploaded