Report Feature: Display All Attachment Types

[OwenPriceSkelly]

Report Feature: Display All Attachment Types

For this issue, implement the following two functions that 'query' an email for information:

val attachment_types : string -> string list
val mbox_attachments : string -> string list

attachment_types takes an email string as an input and outputs a list of the MIME type strings corresponding to all its attachments, e.g. ["application/pdf", "text/plain"]. This should be recursive, so that if the email has parts of parts of parts, and those have attachments, those all go into the output list. Might as well sort it in alphabetical order.

mbox_attachments does the same thing at the MBOX level. So given an MBOX string as an input, it returns a list of all the MIME types that appear anywhere in that input MBOX, in alphabetical order.

They can go in their own module called Report, either in Lib or in a separate file called lib/report.ml. The motivation behind putting them in their own module is that it seems likely we will be incorporating a bunch more "report" features into Attachment Converter, and they will eventually turn into command line options, or perhaps a subcommand.