Don't use File in cmd_upload / cmd_download

It does not look like shell sessions define their own File class,
meaning that the local-platform specific one is always used. Instead
we'll define the separator ourselves since it's all we need to perform
the basic operations necessary to analyze the path string.

lib/msf/base/sessions/command_shell.rb

@@ -425,12 +425,15 @@ Shell Banner:
       return
     end
 
-    if (::File.basename(dst) != File.basename(src))
-      # The destination when downloading is a local file so use this system's separator
-      dst += ::File::SEPARATOR + File.basename(src)
+    fs_sep = platform == 'windows' ? '\\' : '/'
+    if dst.blank?
+      dst = src.split(fs_sep).last
+    elsif ::File.directory?(dst)
+      dst += ::File::SEPARATOR unless dst.end_with?(::File::SEPARATOR)
+      dst += src.split(fs_sep).last
     end
-    dir = ::File.dirname(dst)
-    ::FileUtils.mkdir_p(dir) if dir and not ::File.directory?(dir)
+    dst_dir = ::File.dirname(dst)
+    ::FileUtils.mkdir_p(dst_dir) if dst_dir and not ::File.directory?(dst_dir)
 
     # Get file content
     # match the output style of the Meterpreter equivalent
@@ -438,7 +441,7 @@ Shell Banner:
     content = _file_transfer.read_file(src)
 
     # Write file to local machine
-    File.binwrite(dst, content)
+    ::File.binwrite(dst, content)
     print_status("Completed  : #{src} -> #{dst}")
   end
 
@@ -477,7 +480,8 @@ Shell Banner:
 
     print_status("Uploading  : #{src} -> #{dst}")
     begin
-      content = File.binread(src)
+      # Read file from local machine
+      content = ::File.binread(src)
       _file_transfer.write_file(dst, content)
       print_status("Completed  : #{src} -> #{dst}")
     rescue => e

lib/rex/post/meterpreter/extensions/stdapi/fs/file.rb

@@ -293,7 +293,8 @@ class File < Rex::Post::Meterpreter::Extensions::Stdapi::Fs::IO
   def File.upload(dest, *src_files, &stat)
     src_files.each { |src|
       if (self.basename(dest) != ::File.basename(src))
-        dest += self.separator + ::File.basename(src) unless dest.end_with?(self.separator)
+        dest += self.separator unless dest.end_with?(self.separator)
+        dest += ::File.basename(src)
       end
       stat.call('Uploading', src, dest) if (stat)
 
@@ -350,7 +351,8 @@ class File < Rex::Post::Meterpreter::Extensions::Stdapi::Fs::IO
       if (::File.basename(dest) != File.basename(src))
         # The destination when downloading is a local file so use this
         # system's separator
-        dest += ::File::SEPARATOR + File.basename(src) unless dest.end_with?(::File::SEPARATOR)
+        dest += ::File::SEPARATOR unless dest.end_with?(::File::SEPARATOR)
+        dest += File.basename(src)
       end
 
       # XXX: dest can be the same object as src, so we use += instead of <<