Unfortunately, DataCase renames its stored files as hexadecimal names, so finding a particular file to email it will be nearly impossible if you have a lot of files. If you want to try anyway, you'll first need to locate the DataCase app directory (which is stored, like all iPhone apps with a Universal Unique Identifier (UUID)). The full path to the files is:
Most of the other file storage apps use more standard file naming conventions, and they virtually all store them inside their own Documents folder. I like Air Sharing for everything except Text doc display.
Air Sharing, like most of the file apps, can display many different document types, like pictures, PDFs and text documents. It does most document types quite well, but does a hideous job with plain text documents, because there's no user control over the encoding method or the font used. So, if you need (as I do) to use monospaced fonts to preserve the alignment of text, this app fails miserably. It is one of the few that can display RTF, but unfortunately, it sets the page width too narrowly, destroying the formatting with those as well.