New issue by ashiire on mblaze repository

https://github.com/leahneukirchen/mblaze/issues/248

Description:
When extracting attachments via `mshow`'s `-x` or `-O` flags, any present charset information is lost. This is troublesome, as charset information is difficult to infer from file contents alone.

`mshow` does explicitly mention attachment charset in render mode if a filter is used to render the attachment, like so:
`--- --- --- 3: text/plain size=235 charset="iso-8859-2" render="mshow-plaintext" ---`
However, this seems like the wrong (and inconvenient) place to recover the information from. 

I have a two ideas that might help:
* (Add an option to) explicitly state charset information in list mode, if available.
* Add an option to automatically re-encode extracted attachments to UTF-8, same as in render mode.

I think that either one would be sufficient on its own, but both may be desirable.