Problem Uploading CSS File to Seaside FileLibrary

I am a beginner to Seaside and am currently working through the Dynamic Web Development with Seaside book/tutorial. I'm currently at chapter 17.4, where I need to upload a .png and a .css file to a FileLibrary I created.

I tried uploading them through the Seaside configuration page. The .png uploaded and is working fine. However when I tried the .css file, Seaside gave me this: Error: Improper store into indexable object.

I tried uploading the file programmatically as they demonstrated, and it appeared to work at first, but I noticed it was not displaying and the source code didn't show any references to it in the tag in the browser. When I investigated the file in Pharo, in the method, it was returning as nil.

If anyone can provide and explanation as to why this isn't working, I would really appreciate it. I don't know if it will help but here is the stack trace from the error:

Stack Trace thisContext ByteString(Object)>>error: self '...etc... thisContext ByteString(Object)>>errorImproperStore self '...etc... thisContext ByteString>>at:put: self '...etc... thisContext WriteStream>>nextPut: self a WriteStream thisContext [ :writeStream | | readStream | readStream := aString readStream. [ readStream atEnd ] whileFalse:...etc... self a GRPharoPlatform