Files that have been electronically transferred (e.g. via Internet) may have an unknown header that was added by one of the programs that the sender used when he created and/or transferred the file. The extra bytes at the beginning of the file makes the file unusable because applications cannot recognize the file as an image file. For instance, a GIF file should begin with the letters "GIF87a"or "GIF89a". If this isn't true, the file isn't a valid GIF and cannot be read.
If PMView cannot recognize a file as one of the readable file formats it will automatically scan the file for any of the formats below. Thus PMView will be able to read a GIF or JFIF file with an unknown header (e.g. 3,733 bytes of trash at the beginning of the file).
The following formats can be recovered from files with an unknown header:.
Graphics Interchange Format (GIF)
JPEG File Interchange Format (JPG)
Adobe Photoshop Document (PSD)