Comment by snazz

Here's the relevant frontend source code for the file upload picker, if anyone's wondering (webpack://src/components/exam/submissions/FileInput.js):

  <input
            type="file"
            ref={inputRef}
            name="fileupload"
            disabled={disabled}
            accept={EXTENSIONS[type]}
            data-cb-element="no-cb"
            onChange={async e => {
              const [file] = Object.keys(e.target.files).map(key => e.target.files[key])
              const fileSizeInMb = file.size/(CONVERSION_BASE*CONVERSION_BASE)
              const fileSizeInKb = file.size/CONVERSION_BASE
              const split = file.name.split('.')
              const fileType = split[split.length-1].toLowerCase()
              const extensions = EXTENSIONS[type].split(', ')
              const isAllowedExtension = extensions.includes(`.${fileType}`)
              const alreadyExists = files.find(f => f.name === file.name && f.size === file.size && f.lastModified === file.lastModified)
              let error

              if (alreadyExists)
                error = { title: 'You have already uploaded a file with the same name.', details: 'Please attach a different file.' }
              else if (!isAllowedExtension)
                error = { title: 'This file type is not acceptable.', details: 'Please check the requirements, save your file in one of the accepted formats, and resubmit.' }
              else if (fileSizeInKb <= minSize)
                error = { title: 'Your file is too small.', details: 'Please check the file-size requirements, save a larger version, and resubmit.' }
              else if (fileSizeInMb > maxSize)
                error = { title: 'Your file is too big.', details: 'Please check the file-size requirements, save a smaller version, and resubmit. ' }
            
              if (error)
                await setError(<><strong>{error.title}</strong> {error.details}</>)
              else {
                await updateFiles(file)
                setError(null)
              }

              inputRef.current.value = null
            }}
          />

The EXTENSIONS variable is defined here:

  export const EXTENSIONS = {
    [TYPE_DOC]: '.txt, .doc, .docx, .pdf, .odt',
    [TYPE_PHOTO]: '.png, .jpg, .jpeg',
    [TYPE_AUDIO]: '.m4a, .mp3, .wav, .ogg'
  }