Why not just put service=None into the method signature?

Can you not fit this all on one line?

, optional

I'd like to propose some changes to this logic that will better handle larger files and let Django backends retain control over the saving process a bit better.
First, it's generally better not to read in all the file data at once. Imagine if someone tried to upload a 100MB file as an avatar. We'd try to read it in all at once and swamp the server, crashing things and effectively taking Review Board down. Instead, we can use chunking to build the hash, like so:

uploaded_file = self.cleaned_data['avatar_upload']
file_hash = md5()

for chunk in uploaded_file.chunks():
    file_hash.update(chunk)


For path building, there's a couple things we need to do. Paths are expected to go in as OS-specific paths, and are then normalized in the backend, so we can use os.path.join. We'll want to do this for the default value fo file_path_prefix (I'll cover this in another comment) and we want to build it here, so that we can be sure things work if file_path_prefix doesn't leave off the trailing /.
For saving, we should let Storage.save do its job. This is a bit better than using open and write, and gives the backends a lot more control of the process (and is, I think, actually necessary for some backends). By default, save will ensure we don't end up with a duplicate filename under any circumstances. It will also ensure paths exist, and it will handle chunking (like above). So the rest of this logic in this comment should be:

file_path = storage.save(
    os.path.join(
        self.service.file_path_prefix,
        self.service.get_unique_filename(
            storage.get_valid_filename(filename))),
    uploaded_file)


We're using get_valid_name on the user-provided filename, instead of the resulting path, helping to catch problems, and doing the hash thing there. We're then creating an OS-level path, which storages expect. The result from the method will be the normalized file path that we can return (which always uses "/" for the delimiter, but also takes care of any alterations that need to be made to make the path more correct).
Give that a try. It should result in better future-compatible avatar saving.

We should make sure to add a / if file_path_prefix doesn't end with it.
It's safe to use this as the delimiter for paths in storage. Django will normalize to this on Windows, anyway.

Contents are indented 1 space too far.

We don't use "Returns" for properties.

We should use os.path.join for the uploaded/avatars/ (which also shouldn't have a leading /).

Mind using keyword arguments for this one too?

Typo.

Should just import os. os.path is implied when doing that.

We access the username 3 times. Let's pull it out into a variable to avoid accessor lookups.

No need for the 0.

Ship It!

Minor thing, but for consistency... Most classes use "Unit tests for ..."

Looks unintentional. Blank line should remain.

Ship It!