A file with .fwp file is an archive file created with Microsoft Expression Web and contains all the files and folders for a design. Microsoft Expression Web is a general web design software product by Microsoft and is based on HTML editor. It has been discontinued since 2012 and available free for download by Microsoft. FWP files can be exported and saved as Microsoft’s .cab format.
FWP File Format - More information
Exact file format specifications of FWP files are not available for developer’s reference. However, these are binary files and can be opened in Microsoft Expression Studio that has been discontinued long time ago.