STMF is Stream File. It is stored on the IFS.
For more information.
More specifically: Searching the IBM i 7.1 InfoCenter on the following tokens yielded the below URL along with both the drill-down navigation and a quoted snippet of text:
stream file stmf
A stream file is a randomly accessible sequence of
bytes, with no further structure imposed by the system.
The integrated file system provides support for storing and
operating on information in the form of stream files. Documents that are stored
in your system’s folders are stream files. Other examples of stream files
are PC files and the files in UNIX systems. An integrated file system
stream file is a system object that has an object type of *STMF.