Are you attempting to set a fanout on the writer (in the navigator pane) or on the feature type (in the canvas)?
On the feature type you can set the Raster File Name to an attribute.
The writer fanout should only be used as a fanout for the directory, not the file name.
I think you're going to have to post your workspace so we can look at it. I tried recreating this, but I couldn't, with whatever combination of value I tried. FME always appended .ecw if I didn't, and it didn't seem to have any effect putting extra . characters in the fanout string. I don't see any other reports of this issue either.
I'm trying to set the fanout on the feature type in the canvas. My Feature Type Properties dialog looks different than yours. I do not see a drop-down button beside the Raster File Name input box.
Are you using FME 2014? Check the box Fanout by Attribute and set the dropdown box to the attribute you want to use for the name
I'm using 2015. I updated to 2015.1.3.2 and now the feature properties dialog looks like yours but I still get the error.
I have resolved this. First, I was using RasterGCPClipper Transformer. I changed this to Clipper. Then In the Clipper properties I selected Merge Attributes and chose Merge Clipper, and Use Clippee. I then added a user attribute to the ECW writer and connected the appropriate attribute from the clipper. I then chose this attribute in the Raster File name.
