Updated logic to extract the file extension

[csarnataro]

With some boards (namely ESP32 S3 Dev Kit) the command line coming from the Arduino compute endpoint contains more occurrences of {build.project_name}, e.g. something like {build.project_name}.bootloader.bin [...] {build.project_name}.bin.
In order to support these boards, we have to pick (by convention) the last occurrence, since it's what we use internally to retrieve the right file extension (in this case bin). Otherwise, in the example above, the file extension will be boo from .bootloader.

NOTE: we could even improve this logic using a simple regular expression, something like \{build\.project_name\}\.(\w\w\w)\b, and then extract the extension which matches (\w\w\w)