GPL is a good license for them to release it to you under.
This file is part of Foobar.
Foobar is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
Foobar is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with Foobar. If not, see <http://www.gnu.org/licenses/>.
Basically, when released to you under the GPL (so you don't have to read the whole license like I have) you have the right to modify and redistribute anything under that license. If people don't release to you with a specific license or a legitimate trade mark you are free to do this anyway. However, the GPL will protect you from any possible legal concerns
Also states that they can never charge for the works (then again, neither could you).
There is a lot of information on it, and the only thing that is required of them is to say "We release this under the GPL", and a copy of the GPL in a text file with whatever they send you.
Or you can post it here, with a bit more of a formal wording.
Any content, including but not limited to images, code, patches, names, places, or textures
submitted for use in Synergies MOD for Torchlight 2 (henceforth refereed to as "the mod") becomes
the mutual property of both the developer(s) of "the mod" and the creator of the content.
After the point which the content has been modified or integrated into a revision of "the mod" it becomes
the SOLE property of the developer(s) of "the mod", releasing the original creator from all copyrights and ownership of said content.
All content accepted into "the mod", or otherwise submitted was done so with full acknowledgement of this notice.
All software, images, code, patches, names, places, textures, or any other form of content is provided as is WITHOUT ANY WARRANTY; implied or otherwise.
Or you know, don't do that. It doesn't matter, just trying to help. I generally release my code and whatnot under the GPL so long as it seems appropriate. Use what you got, or either of the options listed.