Improve libmagic MIME-typing solution

[lcjohnso]

Multiple teams have run into issues with the Client's current libmagic based MIME-typing solution for non-image media. In most cases, the libmagic installation was to blame: namely, the need to install of the underlying C library in addition to the python-magic package.

Solution 1: Improve documentation to note the need for C library installation with instructions on how to execute this install. Also consider adding check for presence of C library (e.g., via ctypes.util.find_library('magic')) with explanatory error if one is not found.

Solution 2: Replace libmagic based MIME-typing with another solution. As discussed in #210, options include:

• Use existing library for MIME-typing via extension (e.g., mimetypes)
• Write manual extension-to-MIME mapping function, mirroring that used by the lab's subject uploader in PFE
• ???