Improve backend resolution logic

[vagenas]

Requested feature

Document conversion currently contains a logic for "guessing" / resolving the backend to use for a given input (ref).

This logic has some limitations, e.g. when working with streams, it relies on the first 8KB to detect the backend to use — which may or may not be enough for a correct detection (e.g. deciding info could only appear at the end of a 10KB stream).

Consider ways to remove these limitations.

One possible high-level approach to examine could be to:

• remove the current layer of "guessing" a backend a priori and then committing to that guess, and
• instead, keep for each format, e.g. XML, a list of backends to try one after another, until one successfully parses (can have a default list, parametrizable by the user).