I have a few glass mugs that I love for making homemade lattes that have developed a film on the inside of the cup. The film doesn't affect the taste, but aesthetically is unattractive. I have tried various methods to clean the film from the glass including using rinse agents in the dishwasher, scrubbing the mug with a plastic scrubby and even soaking the mug overnight in vinegar. None of this worked to get rid of the film.
I made a paste of baking soda and water inside the cup and swirled it around, coating the entire inside. If the paste is too runny, it won't stay on the mug, so just add more baking soda if it is to runny. I let it sit for 10 - 15 minutes while I did some other tasks. Then, I scrubbed the inside lightly with a cloth and thoroughly washed the mug.