[12.x] add Attachment::fromUploadedFile method

[rodrigopedra]

Closes #55983

Currently, there is no straightforward way to attach an UploadedFile instance to a mailable.

That is useful on contact forms that only forwards a form to an email.

Attachment::fromData() can be used, but:

• Requires the attachment name to be set manually by the developer
• Requires the developer to remember calling $file->get() to retrieve the uploaded file's content
• Does not set the attachment's mime type from the original file

This PR:

• Adds a new helper Attachment::fromUploadedFile(), which creates an Attachment instance from an UploadedFile instance, and sets its name from the UploadedFile original client name, and sets it mime-type from the inferred mime-type, or from the client mime-type.
• Adds a corresponding test case