BMimeType::GuessMimeType should check for existing file type attribute

[grexe]

From looking at the source of MimeType.cpp, there is room for optimization:

When I just want to get the MIME type of a file which *already has* a filetype attribute set, i.e. BEOS:TYPE with a valid MIME type, it should *not* consult the MIME database again but just return the existing type from the attribute value.

[waddlesplash]

I would expect GuessMimeType to actually guess the type based on contents always. The file's already-set mimetype can be read separately already?

[grexe]

yes that's true, forgot about that. Curiously though, I get different results, e.g. for a Bookmark file:

• BNodeInfo.GetType() gives me application/x-vnd.Be-bookmark,

but

• BMimeType::GuessMimeType gives me application/octet-stream (using the same ref as source)

[pulkomandy]

Bookmarks have no contents (only attributes) so the MIME sniffer cannot guess their type based on the content.

The guessing here intentionally does not use the existing MIMe attribute, this allows for example "mimeset -f" to work.

But there could be a higher level API that calls BNodeInfo.GetType(), and only turns to GuessMimeType if the node has no usable attribute.