I am using the File Library plugin. When I browse to my file and upload it all seems well, except that the file is not there! It never made it to Web Crossing.
The problem is most typically due to the use of a "slash" ( "/" ) in one or more of the folder names that is part of the path you are uploading to. The browser and the server interpret the slash as a normal separator in a directory path and this obviously changes the interpreted destination for the file.
IOW a folder named "test / here" inside a folder named "mine" is seen by the browser and server as "mine/test/here".
There is no such folder named "test" with a subfolder of "here" so the upload gets lost.
To fix, EDIT the folder(s) and remove the slash from the "Filename for URLs:" to read "testHere".
This may also occur with other characters used in directory paths, like periods ( "." )