In CRM direct manages the attachment handling in two ways:
Customer related attachments:
attachments which are from general nature, like contract documents, organizational sheets etc.
Document related attachments:
attachments which belongs directly to the current log/task, like routing sheets, meetings sheets etc. If you remove & archive a meeting such attachments will be restored later.
Allowed attachments are file from type: pdf, xls, ppt, pptx, doc, txt, csv, rtf, png, jpg, jpeg, tiff, html
The document picker collects all 'allowed' file types from a) the local documents directory and b) your Dropbox folders.