Fix attachments code

[jnwarp]

When downloading an attachment, the byte length of the file is appended, see issue #617.

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[gsingh93]

Thanks! What's actually happening here is that readfile is writing to the output buffer directly and the return value is the number of bytes read, which is being stored into $data. $data then gets sent down in the base class (DataController). While readfile is the more efficient way to do things, I think file_get_contents is cleaner in this case.