Some inline attachments should still be seen as attachments

When an attachment is stated as "inline" in the part, WebTop will not show the attachment icon on the grid, because the attachment will be shown inline on the message view, not in the attachment list.

Though, not all file formats will be inlineable in webtop, so for example if I receive a mail with an attached PDF file that is also stated as "inline", that will not show the attachment icon on the grid, even though it will show the attachment on the message view.

We should use the mail Service function "isInlineableMime" to check that the inline part content type is actually inlineable, and still consider it an attachment if it is not.