Return type is now ImportResultThis contains both the real store name (may be different to the filename) and whether the import was successful.
Collision handlers are now called with an additional argument
The filename and real store name are now both passed to the collision handler. See Collision/Conflict Resolution.
FMTCInitialisationException's fields have changed to be more useful in debugging initialisation issues, both internally and externally. If processing these issues manually, you'll need to migrate. See the in-code documentation for more information.
Custom HttpClient Usage
FMTC now supports HTTP/2, through 'http_plus'! HTTP/2 support is enabled by default, with a fallback to HTTP/1.1 (both with a timeout of 5 seconds).
In many of the places where HttpClients where previously accepted as arguments, BaseClient subtypes are now required. To continue using a custom HttpClient, wrap it with an IOClient.