Allow adding media_type to `from_*` class methods flor blobs.

[julianstirling]

Is your feature request related to a problem? Please describe.

If I have a camera that can capture in multiple formats then to follow the current instructions in the docs I would need to create a class for each format just to set the media_type string. Then a function either for each type of image with a specific return, or a large switch statement to decide what class to create.

I can create a blob directly with from_bytes, from_file, from_temporary_directory. Then once it is created change the media type property.

Describe the solution you'd like

Add an optional argument to set the "media_type" to the from_* class methods so I can go:

MEDIA_TYPES = {
    ".jpg": "image/jpeg",
    ".jpeg": "image/jpeg",
    ".png": "image/png",
    ".tiff": "image/tiff",
}

...

@lt.action
def image_as_blob(self):
    filepath = self.capture_to_disk()
    ext = os.path.splitext(filepath)[1]
    media_type = MEDIA_TYPES[ext]
    return lt.blob.Blob.from_file(filepath, media_type=media_type)

Additional context
Add any other context or screenshots about the feature request here.

[rwb27]

I can't see any reason not to add that argument: as you say, it's already possible, it just requires you to set it after creation which is less elegant.

The reason I've encouraged subclassing is that it means your endpoint gets better documentation: the media type of the Blob subclass should (eventually) propagate through to the OpenAPI/Thing Description. IIRC that's not yet implemented, so there is not yet much benefit.

I think I added logic that would allow you to subclass Blob to specify only the first component of the media type, i.e.

class ImageBlob(lt.Blob):
    media_type = "image/*"

That would then allow the endpoint (in the future) to be described as returning an image, without specifying the precise format.

Figuring out a nice way to let the endpoint say "I return one of these four mimetypes" would be a nice challenge for the future. On the other hand, it would make a lot more sense once there's a way for the action to return a result directly (i.e. avoid polling) for quick actions.

[rwb27]

It was also my intention that, at some point in the near future, LabThings would provide a useful set of pre-made Blob subclasses, for example ImageBlob could then gain a method to reconstitute itself into a PIL image, and maybe even a from_image method.