At The BMJ, we use a system called ScholarOne to manage our submission processes. Essentially, ScholarOne will convert your manuscript to a PDF for the review process. Most common word processing formats are accepted for text and tables, although the system prefers Microsoft Word, and images should be submitted as GIF, TIFF, EPS, or JPEG files.
The system can also accept supplementary files (for example: videos, datasets, research protocols, and checklists or statements), related articles published or available elsewhere, articles in press elsewhere, permission letters, etc. These are files that normally do not appear with the print article, although they might accompany the final version of the paper online. Supplementary files are not converted to PDF but will be available to reviewers or editors exactly as you upload them.