Investigate MULTIPART file attachments

[shrsv]

With the changes related to l2config.env, I think MULTIPART attachments have gone broken. The MULTIPART attachment path must always be calculated relative the relevant .l2 file.

Moreover, in the environment loading logic, we seem to be making an error, that is chdir, load up env, chdir, go on parsing, making request, etc. The second chdir probably should happen as the last action.

@lovestaco