Allow configuration of UploadFS

[gerricom]

I ran into a little problem with our current hoster. I'm using the german uberspace project for hosting a Rocket.Chat instance for our little team of seven users. Everything runs quite smoothly in this environment but file uploads are an issue here.

UploadFS uses in its standard configuration a temporary dir located at /tmp/ufs/.... Uberspace encourages you to use a local tmp dir instead because nobody knows, what the global tmp dir can end up. So as sometimes file uploads are stuck, I took a close look. UploadFS allows you to set a custom tmp dir (UploadFS.config.tmpDir). For testing purposes, I changed this hardcoded in the initialization of ufs, but it would be nicer, to have a option to change this, if one needs to change it.

[luto]

@rodrigok can you please outline what an acceptable solution would like? what do you think about respecting $TMPDIR here?

[d-sko]

If @luto's suggestion is acceptable, I can make a pull request for it.

[geekgonecrazy]

Seems acceptable. Hopefully we can get merged