These were the official guidelines on the government application site:
1. Document type: On completing your on line application you will be advised of the mandatory supporting evidence UKVI will need to access your application. You must upload all the mandatory supporting evidence, we will not be able to submit your application if mandatory information is missing. Further supporting documents guidance is on gov.uk for the documents required for your application route.
2. Category: Your supporting evidence needs to be submitted under the categories advised, our upload facility allows for this and having the right documents in the right category will make your evidence easier to find and review, so we suggest that you organise and upload your supporting documents in the correct categories.
3. File Format: Please upload documents in PDF (preferred), JPG and PNG format.
4. File Name: needs to be as simple as possible e.g. bank-statement-Apr-2018.pdf Do not upload files with file names having special characters, #,ë, or bank.statement.apr.2018.pdf
5. File Size: each uploaded document has a file size limit of 6MB, uploading larger files will result in your documents being rejected, so please ensure the file is as small as possible and not more than 6 MB. If you document is likely to be over 6MB please split into parts and create separate files. There is no limit on the number of documents/files you can upload.
6. Scan in Greyscale or Black and white: To help contain the files size to 6MB please scan in ‘Greyscale’ or ‘Black and white’ and not in ‘Colour’. It is also helpful to keep the resolution between 150 dpi (dots per inch) to 300 dpi. This can be achieved by configuring your device by the available ‘Settings’ option, that allows you to choose the file format, colour and the dpi resolution. With this approach, you will be able to scan up to 50 pages per document within the allowed 6 MB limit.
7. Password: Please do not upload password protected files as the caseworker will not be able to open them for assessment and therefore your application may be delayed."