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. For Mandriva 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.