Update filename on werkzeug FileStorage for uploaded files

This is an issue for fields whose name might be changed at any point, like any image ending in "jpeg" (which will be renamed to "jpg". That would cause the actual file to be saved to disk with ".jpeg", but be saved elsewhere as ".jpg".

This example (mostly copied from the example in `examples/forms/simple.py`) shows it better than I can explain:

    import os
    from flask import Flask
    from flask.ext.admin import Admin, form
    from flask.ext.admin.contrib import sqla
    from flask.ext.sqlalchemy import SQLAlchemy

    app = Flask(__name__)
    app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///hello.db'
    db = SQLAlchemy(app)

    os.path.isdir('static') or os.mkdir('static')

    class Photo(db.Model):
        id = db.Column(db.Integer, primary_key=True)
        path = db.Column(db.Unicode(128))

    class PhotoView(sqla.ModelView):
        form_extra_fields = {'path': form.ImageUploadField('Photo', base_path='static')}

    admin = Admin(app, 'Photos are broken!')
    admin.add_view(PhotoView(Photo, db.session))

    db.create_all()
    app.run(debug=True)

... try uploading files with `.jpeg` vs `.jpg` extensions. It's pretty broken.
Ignore case for allowed extensions

Fixes error where 'image.JPG' will be disallowed when `form.upload.FileUploadField.allowed_extensions` is set to `('jpg',)`, for example.
Raise ImportError instead of Exception for an import error

The issue at hand is in fact an error on import, and more specific exceptions are nicer to work with.
Merge pull request #465 from uniphil/imageupload

File and ImageUploadField fixes