Add explicit file MIME-type when writing with GridFS backend

[rclement]

When using the GridFS backend and trying to serve files with it, the MIME-type cannot be found by Flask on the returned file. When writing a file to GridFS, the MIME-type should be included allowing to retrieve it when serving the file.

Run the following sample app to reproduce the issue:

from flask import Flask, jsonify, request
import flask_fs as fs
import os
from uuid import uuid4

app = Flask(__name__)
app.config.update(
    FS_BACKEND=os.environ.get('STORAGE_BACKEND'),
    FS_GRIDFS_MONGO_URL=os.environ.get('MONGO_URL'),
    FS_GRIDFS_MONGO_DB=os.environ.get('MONGO_DB')
)

uploads = fs.Storage('uploads')
fs.init_app(app, uploads)

@app.route('/upload', methods=['POST'])
def upload():
    content = request.files.get('content')
    if not content or content.filename == '':
        abort(HTTPStatus.BAD_REQUEST)

    filename = str(uuid4())
    uploads.write(filename, content)
    upload_url = uploads.url(filename)
    return jsonify({
        'upload_url': upload_url
    })

if __name__ == '__main__':
    app.run(debug=True)

$ export STORAGE_BACKEND="gridfs"
$ export MONGO_URL="mongodb://localhost:27017"
$ export MONGO_DB="uploads"
$ python app.py

Perform the following requests to observe the issue:

$ http -f POST localhost:5000/upload content@<path/to/file.{png,jpg}>
HTTP/1.0 200 OK
Access-Control-Allow-Origin: *
Content-Length: 68
Content-Type: application/json
Date: Wed, 11 Apr 2018 13:14:41 GMT
Server: Werkzeug/0.14.1 Python/3.6.5

{
    "upload_url": "/uploads/c5c195f1-41a9-41ad-a923-0111e51d4127"
}

$ http GET localhost:5000/uploads/c5c195f1-41a9-41ad-a923-0111e51d4127
HTTP/1.0 500 INTERNAL SERVER ERROR
Connection: close
Content-Type: text/html; charset=utf-8
Date: Wed, 11 Apr 2018 13:15:03 GMT
Server: Werkzeug/0.14.1 Python/3.6.5
X-XSS-Protection: 0

127.0.0.1 - - [11/Apr/2018 15:28:59] "GET /uploads/c5c195f1-41a9-41ad-a923-0111e51d4127 HTTP/1.1" 500 -
Traceback (most recent call last):
  File "/lib/python3.6/site-packages/flask/app.py", line 1997, in __call__
    return self.wsgi_app(environ, start_response)
  File "/lib/python3.6/site-packages/flask/app.py", line 1985, in wsgi_app
    response = self.handle_exception(e)
  File "/lib/python3.6/site-packages/flask/app.py", line 1540, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/lib/python3.6/site-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/lib/python3.6/site-packages/flask/app.py", line 1982, in wsgi_app
    response = self.full_dispatch_request()
  File "/lib/python3.6/site-packages/flask/app.py", line 1614, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/lib/python3.6/site-packages/flask/app.py", line 1517, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/lib/python3.6/site-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/lib/python3.6/site-packages/flask/app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "/lib/python3.6/site-packages/flask/app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/lib/python3.6/site-packages/flask_fs/views.py", line 17, in get_file
    return storage.serve(filename)
  File "/lib/python3.6/site-packages/flask_fs/storage.py", line 354, in serve
    return self.backend.serve(filename)
  File "/lib/python3.6/site-packages/flask_fs/backends/gridfs.py", line 78, in serve
    return send_file(file, mimetype=file.content_type)
  File "/lib/python3.6/site-packages/flask/helpers.py", line 527, in send_file
    'Unable to infer MIME-type because no filename is available. '
ValueError: Unable to infer MIME-type because no filename is available. Please set either `attachment_filename`, pass a filepath to `filename_or_fp` or set your own MIME-type via `mimetype`.

Packages versions
Flask (0.12.2)
flask-fs (0.6.0)

[coveralls]

Coverage increased (+0.02%) to 94.744% when pulling 7b58ff6 on rclement:gridfs-fix-serve-contenttype into 19a895b on noirbizarre:master.

[noirbizarre]

Test for value not None, I'll merge it, but I think it won't solve the fact that this value is almost never set by Werkzeug

[noirbizarre]

You should test that it's not None because most of the time Werkzeug set it to None (never succed to have another value).

if getattr(content, 'content_type'):

[rclement]

Ok, done. On my end, I always get a proper "content_type" in "request.files" if the originating request is set properly (i.e. the "Content-Type" header is set, thus making Werkzeug guess it).

[noirbizarre]

Thanks! I'll probably release it today.

[rclement]

Oh nice! Thanks a lot!