Are you adding these files from within the Chrome extension? As that is not possible, Google does not allow extensions to upload files. There is also a problem with the Mac desktop app, but drag and drop still works.
Please let me know if that is not the case, and what kind of file you are trying to upload, and if you receive any error messages.
I am having this issue as well. I am attaching an excel spreadsheet using Google Chrome, but the document is not uploading correctly. Sheet 1 is the only sheet that remains inside of the document and even on that sheet, all of the formatting and formulas are gone.
Our upload feature does not modify any parts of the uploaded document in any way. That said, please clarify - you export a Google Document, save it as an Excel file, when you open it locally on your computer it's OK, but if you upload the same file to Todoist and then download it from Todoist, it's broken?
How large is the file? More than 20 MB?
If that's not the case and you're actually uploading an Excel sheet into Google Drive, then that's unfortunately something beyond our control, we're not affiliated with Google and have no influence on the working and development of Google Drive.