Upload errors are almost always caused by the file itself. Please check:

File size — each file must be smaller than :maxsize MB. Photos and videos taken with a modern phone often exceed this — resize the photo or export the video in a smaller resolution before uploading.
File type — supported photo formats are JPG and PNG; supported video formats are MP4, MOV, M4V, MPEG, WMV and AVI. Some phones save photos as HEIC — convert them to JPG first.
Video length — very long videos may be rejected; try trimming them or splitting them into parts.
Connection — on a slow or unstable connection, upload files one at a time.

If the upload still fails, contact our support and include the file type and its size — we will help you out.