Prefer file extension for detecting file compression.

     Change: extension-based-compression-detection
     Purpose: Prefer file extension for detecting file compression.
     Reason: RPM tries to detect file compression by inspecting
                     "magic" bytes at the front of a file. This works
                     for formats with known "magic" bytes just fine,
                     but fails horribly for formats like LZMA where no
                     such "magic" bytes exist. As a result RPM 5 this
                     way too often thinks .tar files are LZMA compressed
                     and stumbles over this during build-time. For
                     OpenPKG we prefer to first use file extension
                     based determination (if someone is mis-names
                     his distribution files we have to take action
                     anyway) and fall back to "magic" byte based
                     determination only if no well known extension is
                     found. Additionally, detecting LZMA we want to do
                     via file extension only.