The example at yaws web site to read the file upload is a good starting point but it's too simplistic. I extended the example so it's useful in the real world. (Update: Thanks to Steve Vinoski, this module(yaws_multipart) is now part of the yaws git tree).

1. It reads all parameters – files uploaded and other simple parameters .
2. It takes a few options to help file uploads. Specifically:
   1. {max_file_size, MaxBytes} : If file exceeds MaxBytes bytes, return an error
   2. no_temp_file: read the uploded file in memory without any temp files
   3. {temp_file,FullFilePath}: Specify full path for the temp file. If not given, a unique file name is generated
   4. {temp_dir, TempDir} : Specify a directory to store uploaded temp file. By default '/tmp' is used.

Using it is simple. Just call read_multipart_form from your 'out' function and it'll return a tuple with first element either 'get_more', 'done' or 'error'. The 'get_more' implies more data needs to be read and you must call read_multipart_form again. 'done' implies it's done reading all parameters and you're free to proceed. The 'done' tuple also returns a 'dict' full of params. This dict can be queried for parameters by name. For file upload parameters it returns one of the following lists:

[{filename, "name of the uploaded file as entered on the form"},
  {value, Contents_of_the_file_all_in_memory}]
OR
[{filename, "name of the uploaded file as entered on the form"},
  {temp_file, "full pathname of the temp file"}]

In the second case, it's your responsibility to remove the temp file. Usage example:

-module(my_yaws_controller).
-export([out/1]).
 
out(Arg) ->
     Options = [no_temp_file],
     case yaws_multipart:read_multipart_form(Arg, Options) of
             {done, Params} -> 
                   io:format("Params : ~p",[Params]),
                   [{filename, File_name},{value,File_content}] = dict:find("my_file", Params),
                  Another_param = dict:find("another_param", Params);
                  % do something with File_name, File_content and Another_param
              {error, Reason} ->
                   io:format("Error reading multipart form: ~s", [Reason]);
              Other -> Other
      end
.