Update the documentation on how to import attachments using CSV

The documentation about importing attachments using CSV (https://confluence.atlassian.com/display/JIRA/How+to+import+attachment+using+CSV) is not clear explaining that the file path used is the File URI scheme, and hence, it won't change on different file systems path formats (e.g. using backslash on Windows systems instead of slashes).

It would be good to include this information in the document, and also the limitations which are implied from this (e.g. file names with spaces will have issues).